diff --git a/main.lua b/main.lua
index 2d0b76a..245a2e9 100644
--- a/main.lua
+++ b/main.lua
@@ -35,6 +35,9 @@ function love.load()
 	_initAccessibility()
 	gameWorld = World:new()
 	_initLevelEditor()
+	-- getAllAssets runs in _initLevelEditor; deserialize the world only after
+	-- its asset metadata exists, otherwise every non-player object is skipped.
+	gameWorld:loadLevels()
 
 	_initCanvases()
 
@@ -43,10 +46,14 @@ function love.load()
 	-- Resume the game: prefer the runtime autosave so the last saved position is
 	-- restored, falling back to the authored source level. Editor room-opens go
 	-- through their own (source-only) load path, so this only affects resume.
-	currentRoom = Room:new{
-		name = gameWorld.lastRoom or "start",
-		runtime = true
-	}
+	local initialRoom = gameWorld.lastRoom or "start"
+	-- Preserve the old resume behavior for the active room while the rest of the
+	-- world comes from the authored level files.
+	currentRoom = Room:new{name = initialRoom, runtime = true}
+	if gameWorld.levels[initialRoom] then
+		gameWorld.levels[initialRoom] = currentRoom
+		gameWorld:rebuildCollision()
+	end
 	gameWorld:setLastRoom(currentRoom.name)
 	setEditorRoomName(currentRoom.name)
 
@@ -69,7 +76,7 @@ function love.update(dt)
 		Input:stopMoveRepeat()
 	else
 		local repeatedMove = Input:handler(dt)
-		if repeatedMove and not Dialogue.isOpen() then currentRoom:movePlayer(repeatedMove) end
+		if repeatedMove and not Dialogue.isOpen() then gameWorld:movePlayer(currentRoom, repeatedMove) end
 	end
 	currentRoom:update(dt)
 
@@ -164,10 +171,10 @@ function love.keypressed(key)
 
 	if editorView ~= "world" and Input.isPlayerMove(key) then
 		Input:beginMove(key)
-		currentRoom:movePlayer(key)
+		gameWorld:movePlayer(currentRoom, key)
 	elseif editorView ~= "world" and Input.isPlayerSwitch(key) then
 		local c = Input.playerColor(key)
-		currentRoom:switchPlayerColor(c)
+		gameWorld:switchPlayerColor(currentRoom, c)
 	end
 
 	-- if key == "w" then
diff --git a/room.lua b/room.lua
index d354618..fe5531f 100644
--- a/room.lua
+++ b/room.lua
@@ -132,6 +132,7 @@ function Room:draw()
 	for _, channel in ipairs(self.colorsPlayerHas) do
 		if self.player[channel] then self.player[channel]:draw(channel) end
 		for _, entity in pairs(self.collideables[channel]) do entity:draw(channel) end
+		if gameWorld and gameWorld.levels and gameWorld.levels[self.name] == self then gameWorld:drawOverlaps(self, channel) end
 		for _, blocker in pairs(self.playerBlockers[channel]) do blocker:draw(channel) end
 		for _, switch in pairs(self.switches[channel]) do switch:draw(channel) end
 		for _, npc in ipairs(self.npcs) do npc:draw(channel) end
@@ -181,7 +182,7 @@ function Room:createEntityMatrix(entities)
 	for _, entity in pairs(entities) do
 		for _, occupiedCells in pairs(entity:getOccupiedCells()) do
 			--we store a table reference so we can call a method on it during a chec_
-			matrix[occupiedCells.y][occupiedCells.x] = entity
+			if self:isInBounds(occupiedCells) then matrix[occupiedCells.y][occupiedCells.x] = entity end
 		end	
 	end
 	-- printMatrix(matrix)
@@ -193,7 +194,9 @@ function Room:getCellInMatrix(matrix, cell)
 	return matrix[cell.y][cell.x]
 end
 
-function Room:movePlayer(key)
+-- Kept for isolated room tests/editor previews. The live game goes through
+-- World:movePlayer so no gameplay decision is room-local.
+function Room:movePlayerLocal(key)
 	
 	local moved = false
 	local previousPositions = {} --these are so we can perform the overlap chec_
@@ -224,23 +227,36 @@ function Room:movePlayer(key)
 			x = pos.x + dir.x, y = pos.y + dir.y
 		} -- this is for reference for each color as we change the targetCells table
 
+		local useWorld = gameWorld and gameWorld.levels and gameWorld.levels[self.name] == self
+		local context = useWorld and gameWorld:movementContext(self, color)
+		local worldTarget = useWorld and context:worldCell(initialTargetCell)
 		local initialTargetEntity
 
-		if self:isInBounds(initialTargetCell) then
+		if useWorld then
+			initialTargetEntity = context:entityAt(worldTarget)
+		elseif self:isInBounds(initialTargetCell) then
 			initialTargetEntity = self:getCellInMatrix(self.collidableMatrices[color], initialTargetCell)
 		end
 
 		if initialTargetEntity then initialTargetEntity:bump() end
 		local playerBlocker
-		if self:isInBounds(initialTargetCell) and self.playerBlockerMatrices and self.playerBlockerMatrices[color] then
+		if useWorld then
+			playerBlocker = context:playerBlockerAt(worldTarget)
+		elseif self:isInBounds(initialTargetCell) and self.playerBlockerMatrices and self.playerBlockerMatrices[color] then
 			playerBlocker = self:getCellInMatrix(self.playerBlockerMatrices[color], initialTargetCell)
 		end
 
-		if not self:isInBounds(initialTargetCell) then
+		if useWorld and not context:isInBounds(worldTarget) then
+			move = true
+		elseif not useWorld and not self:isInBounds(initialTargetCell) then
 			-- let's us move out of bounds to Change Rooms without pulling a bloc_
 			move = true
 		elseif playerBlocker and playerBlocker:blocksPlayer() then
 			move = false
+		elseif useWorld then
+			move, entitiesToPush = Movement.findMovableGroup({worldTarget}, dir,
+				function(cell) return context:entityAt(cell) end,
+				function(cell) return context:isInBounds(cell) end)
 		else
 			move, entitiesToPush = Movement.findMovableGroup(
 				{initialTargetCell}, dir,
@@ -253,7 +269,11 @@ function Room:movePlayer(key)
 
 		local initialTargetCell = {x = pos.x - dir.x, y = pos.y - dir.y}
 		local pull = false
-		if self:isInBounds(initialTargetCell) then
+		if useWorld and context:isInBounds(context:worldCell(initialTargetCell)) then
+			pull, entitiesToPull = Movement.findMovableGroup({context:worldCell(initialTargetCell)}, dir,
+				function(cell) return context:entityAt(cell) end,
+				function(cell) return context:isInBounds(cell) end)
+		elseif not useWorld and self:isInBounds(initialTargetCell) then
 			pull, entitiesToPull = Movement.findMovableGroup(
 				{initialTargetCell}, dir,
 				function(cell) return self:getCellInMatrix(self.collidableMatrices[color], cell) end,
@@ -262,10 +282,15 @@ function Room:movePlayer(key)
 		end
 
 		if move then
-			for _, entity in pairs(entitiesToPush) do
-				entity:move(axis, dir[axis])
+			if useWorld then
+				local group = {}
+				for entity in pairs(entitiesToPush) do group[entity] = entity end
+				if pull then for entity in pairs(entitiesToPull) do group[entity] = entity end end
+				gameWorld:moveGroup(group, dir)
+			else
+				for _, entity in pairs(entitiesToPush) do entity:move(axis, dir[axis]) end
 			end
-			if pull == true then
+			if pull == true and not useWorld then
 				for _, entity in pairs(entitiesToPull) do
 					-- A connected component can wrap around the player and be found by
 					-- both searches. Every entity still moves exactly one grid cell.
@@ -281,12 +306,20 @@ function Room:movePlayer(key)
 	end
 
 	if moved then
-		self:playerJoinCheck(previousPositions, newPositions)
+		if useWorld then gameWorld:playerJoinCheck(self, previousPositions, newPositions)
+		else self:playerJoinCheck(previousPositions, newPositions) end
 		self:tick()
 	end
 end
 
-function Room:switchPlayerColor(color)
+function Room:movePlayer(key)
+	if gameWorld and gameWorld.levels and gameWorld.levels[self.name] == self then
+		return gameWorld:movePlayer(self, key)
+	end
+	return self:movePlayerLocal(key)
+end
+
+function Room:switchPlayerColorLocal(color)
 	-- Number keys can request a colour the player has not collected yet.  Do
 	-- nothing in that case: there is no corresponding player object to switch
 	-- to (or use as the sound source).
@@ -304,6 +337,13 @@ function Room:switchPlayerColor(color)
 	return canSwitch
 end
 
+function Room:switchPlayerColor(color)
+	if gameWorld and gameWorld.levels and gameWorld.levels[self.name] == self then
+		return gameWorld:switchPlayerColor(self, color)
+	end
+	return self:switchPlayerColorLocal(color)
+end
+
 function Room:setActiveColor(color, bool)
 	self.activeColors[color] = bool
 end
@@ -361,7 +401,7 @@ function Room:onlyColorIsActive(color)
 	end
 end
 
-function Room:tick()
+function Room:tickLocal()
 	for color, entityArray in pairs(self.collideables) do
 		for _, entity in pairs(entityArray) do
 			entity:tick()
@@ -371,6 +411,11 @@ function Room:tick()
 	self:createPlayerBlockerMatrix()
 	self:createSwitchMatrix()
 	self:resolveSwitchesAndDoors()
+end
+
+function Room:tick()
+	if gameWorld and gameWorld.levels and gameWorld.levels[self.name] == self then return gameWorld:tick(self) end
+	self:tickLocal()
 	self:nextRoomCheck()
 end
 
@@ -445,7 +490,14 @@ function Room:switchCheck()
 	for color, switchArray in pairs(self.switches) do
 		local onSwitches = {}
 		for _, switch in pairs(switchArray) do
-			local targetCell = self:getCellInMatrix(self.collidableMatrices[color], switch:getGridPos())
+			local targetCell
+			if gameWorld and gameWorld.levels and gameWorld.levels[self.name] == self then
+				local cell = gameWorld:worldCell(self, switch:getGridPos())
+				local row = cell and gameWorld.collidableMatrices[color][cell.y]
+				targetCell = row and row[cell.x]
+			else
+				targetCell = self:getCellInMatrix(self.collidableMatrices[color], switch:getGridPos())
+			end
 			local player = self.player[color]
 			-- Players are not stored in collidableMatrices, but a colour copy
 			-- standing on its matching switch presses it just like a block does.
@@ -769,6 +821,7 @@ function Room:serialize(includePlayers)
 end
 
 function nextRoom(pos)
+	if gameWorld and currentRoom then return gameWorld:nextRoom(currentRoom, pos) end
 	local exit
 	local newPos = {x = pos.x, y = pos.y}
 	if pos.x > gridWidth then
@@ -791,13 +844,17 @@ function nextRoom(pos)
 
 	-- Missing map neighbours intentionally wrap back into the room just exited.
 	-- A placed neighbour gets the same carried player state at its opposite edge.
-	currentRoom = Room:new{
-		name = nextName or currentName,
-		runtime = true,
-		player = {
-			x = newPos.x, y = newPos.y, activeColors = currentRoom:getActiveColors()
-		}
-	}
+	local playerState = {x = newPos.x, y = newPos.y, activeColors = currentRoom:getActiveColors()}
+	currentRoom = gameWorld and gameWorld.levels[nextName or currentName] or Room:new{name = nextName or currentName, runtime = true}
+	if currentRoom then
+		currentRoom.activeColors = {red = false, green = false, blue = false}
+		for _, color in ipairs(playerState.activeColors) do
+			if currentRoom.player[color] then currentRoom.player[color].x, currentRoom.player[color].y = playerState.x, playerState.y
+			else currentRoom:registerPlayer(Player:new(playerState.x, playerState.y, color), color) end
+			currentRoom:setActiveColor(color, true)
+		end
+		gameWorld:playerJoinCheck(currentRoom)
+	end
 	if gameWorld then gameWorld:setLastRoom(currentRoom.name) end
 	if currentRoom.name then setEditorRoomName(currentRoom.name) end
 
diff --git a/world.lua b/world.lua
index 7a4abe7..8c8aeaf 100644
--- a/world.lua
+++ b/world.lua
@@ -1,5 +1,7 @@
 World = class("World")
 
+local Movement = require "movement"
+
 -- The world is deliberately only a layout: rooms continue to own their puzzle
 -- state.  Keeping the two separate lets the editor move a room without
 -- rewriting its level file.
@@ -10,6 +12,13 @@ function World:initialize()
 	self.previewCache = {}
 	self.lastRoom = nil
 	self:load()
+	self.levels = {}
+
+end
+
+function World:loadLevels()
+	for _, placed in ipairs(self.rooms) do self.levels[placed.name] = Room:new{name = placed.name} end
+	self:rebuildCollision()
 end
 
 function World:listRoomNames()
@@ -128,6 +137,292 @@ function World:getNeighbor(name, exit)
 	return self:roomAt(room.x + direction.x, room.y + direction.y)
 end
 
+-- All placed levels share these sparse matrices.  Rendering still uses the
+-- selected Room, whose ordinary local matrices remain unchanged.
+function World:rebuildCollision()
+	self.collidableMatrices = {red = {}, green = {}, blue = {}}
+	self.playerBlockerMatrices = {red = {}, green = {}, blue = {}}
+	self.entityRooms = {}
+	local function put(matrix, cell, entity)
+		matrix[cell.y] = matrix[cell.y] or {}
+		matrix[cell.y][cell.x] = entity
+	end
+	for name, room in pairs(self.levels) do
+		local location = self:roomLocation(name)
+		if location then
+			for color, entities in pairs(room.collideables) do
+				for _, entity in pairs(entities) do
+					self.entityRooms[entity] = room
+					for _, cell in ipairs(entity:getOccupiedCells()) do
+						put(self.collidableMatrices[color], {
+							x = (location.x - 1) * gridWidth + cell.x,
+							y = (location.y - 1) * gridHeight + cell.y,
+						}, entity)
+					end
+				end
+			end
+			-- NPCs are local room state, but globally collidable: a movement lookup
+			-- must find them so its normal bump() path can open dialogue.
+			for _, npc in ipairs(room.npcs) do
+				self.entityRooms[npc] = room
+				for _, cell in ipairs(npc:getOccupiedCells()) do
+					local worldCell = {
+						x = (location.x - 1) * gridWidth + cell.x,
+						y = (location.y - 1) * gridHeight + cell.y,
+					}
+					for color in pairs(self.collidableMatrices) do put(self.collidableMatrices[color], worldCell, npc) end
+				end
+			end
+			for color, entities in pairs(room.playerBlockers) do
+				for _, entity in pairs(entities) do
+					for _, cell in ipairs(entity:getOccupiedCells()) do
+						put(self.playerBlockerMatrices[color], {
+							x = (location.x - 1) * gridWidth + cell.x,
+							y = (location.y - 1) * gridHeight + cell.y,
+						}, entity)
+					end
+				end
+			end
+		end
+	end
+end
+
+function World:worldCell(room, cell)
+	local location = self:roomLocation(room.name)
+	return location and {x = (location.x - 1) * gridWidth + cell.x, y = (location.y - 1) * gridHeight + cell.y}
+end
+
+function World:roomCell(cell)
+	local roomX = math.floor((cell.x - 1) / gridWidth) + 1
+	local roomY = math.floor((cell.y - 1) / gridHeight) + 1
+	local name = self:roomAt(roomX, roomY)
+	local room = name and self.levels[name]
+	return room, {x = cell.x - (roomX - 1) * gridWidth, y = cell.y - (roomY - 1) * gridHeight}
+end
+
+function World:movementContext(room, color)
+	local proxies = {}
+	local world = self
+	local context = {}
+	function context:worldCell(cell) return world:worldCell(room, cell) end
+	function context:isInBounds(cell) return world:roomCell(cell) ~= nil end
+	function context:entityAt(cell)
+		local row = world.collidableMatrices[color][cell.y]
+		local entity = row and row[cell.x]
+		if not entity then return nil end
+		if proxies[entity] then return proxies[entity] end
+		local owner = world.entityRooms[entity]
+		local position = world:worldCell(owner, entity:getGridPos())
+		local proxy = {entity = entity, room = owner, color = color, x = position.x, y = position.y}
+		function proxy:canMove() return self.entity:canMove() end
+		function proxy:canPassOver() return self.entity:canPassOver() end
+		function proxy:isSticky() return self.entity:isSticky() end
+		function proxy:isSkewer() return self.entity:isSkewer() end
+		function proxy:isSpiky() return self.entity:isSpiky() end
+		function proxy:bump() return self.entity:bump() end
+		function proxy:getOccupiedCells(offset)
+			offset = offset or {x = 0, y = 0}
+			local cells = {}
+			for _, shapeCell in ipairs(self.entity:getCellShape()) do
+				table.insert(cells, {x = self.x + shapeCell.x - 1 + offset.x, y = self.y + shapeCell.y - 1 + offset.y})
+			end
+			return cells
+		end
+		proxies[entity] = proxy
+		return proxy
+	end
+	function context:playerBlockerAt(cell)
+		local row = world.playerBlockerMatrices[color][cell.y]
+		return row and row[cell.x]
+	end
+	return context
+end
+
+-- Player copies live in room files, but joining is a world-coordinate rule.
+-- This also lets a copy rejoin immediately when the player crosses into its
+-- room, rather than waiting for one more local move.
+function World:playerJoinCheck(room, previousPositions, newPositions)
+	local activeColors = room:getActiveColors()
+	local deactivate, activate = {}, {}
+	if previousPositions and newPositions and #activeColors > 1 then
+		for _, color in ipairs(activeColors) do
+			if Entity.positionsAreEqual(previousPositions[color], newPositions[color]) then
+				table.insert(deactivate, color)
+			end
+		end
+	end
+	for color, player in pairs(room.player) do
+		if not room:colorIsActive(color) then
+			local playerCell = self:worldCell(room, player:getGridPos())
+			local overlaps = playerCell and #activeColors > 0
+			for _, activeColor in ipairs(activeColors) do
+				local activeCell = self:worldCell(room, room.player[activeColor]:getGridPos())
+				if not activeCell or activeCell.x ~= playerCell.x or activeCell.y ~= playerCell.y then overlaps = false; break end
+			end
+			if overlaps then table.insert(activate, color) end
+		end
+	end
+	for _, color in ipairs(activate) do room:setActiveColor(color, true) end
+	for _, color in ipairs(deactivate) do room:setActiveColor(color, false) end
+end
+
+function World:movePlayer(room, key)
+	local directions = {
+		up = {x = 0, y = -1, axis = "y"}, down = {x = 0, y = 1, axis = "y"},
+		left = {x = -1, y = 0, axis = "x"}, right = {x = 1, y = 0, axis = "x"},
+	}
+	local dir = directions[key]
+	if not dir then return end
+	local moved, previous, current = false, {}, {}
+	for _, color in ipairs(room:getActiveColors()) do
+		local player = room.player[color]
+		local pos = player:getGridPos()
+		previous[color] = pos
+		local context = self:movementContext(room, color)
+		local target = context:worldCell({x = pos.x + dir.x, y = pos.y + dir.y})
+		local targetEntity = context:entityAt(target)
+		if targetEntity then targetEntity:bump() end
+		local blocker = context:playerBlockerAt(target)
+		local canMove, pushing = true, {}
+		if context:isInBounds(target) and not (blocker and blocker:blocksPlayer()) then
+			canMove, pushing = Movement.findMovableGroup({target}, dir,
+				function(cell) return context:entityAt(cell) end,
+				function(cell) return context:isInBounds(cell) end)
+		elseif blocker then
+			canMove = false
+		end
+
+		local pulling, pullingGroup = false, {}
+		local behind = context:worldCell({x = pos.x - dir.x, y = pos.y - dir.y})
+		if context:isInBounds(behind) then
+			pulling, pullingGroup = Movement.findMovableGroup({behind}, dir,
+				function(cell) return context:entityAt(cell) end,
+				function(cell) return context:isInBounds(cell) end)
+		end
+		if canMove then
+			local group = {}
+			for entity in pairs(pushing) do group[entity] = entity end
+			if pulling then for entity in pairs(pullingGroup) do group[entity] = entity end end
+			self:moveGroup(group, dir)
+			player:move(dir.axis, dir[dir.axis])
+			moved = true
+		end
+		current[color] = player:getGridPos()
+	end
+	if moved then
+		self:playerJoinCheck(room, previous, current)
+		self:tick(room)
+	end
+end
+
+function World:switchPlayerColor(room, color)
+	local player = room.player[color]
+	if not player then return false end
+	local activeColors = room:getActiveColors()
+	local canSwitch = #activeColors ~= 1 or activeColors[1] ~= color
+	if canSwitch then
+		for _, possibleColor in ipairs(room:getPossibleColors()) do
+			room:setActiveColor(possibleColor, possibleColor == color)
+		end
+	end
+	player:playSwitchSound(canSwitch)
+	return canSwitch
+end
+
+function World:tick(room)
+	room:tickLocal()
+	self:rebuildCollision()
+	self:nextRoomCheck(room)
+end
+
+function World:nextRoomCheck(room)
+	for _, color in ipairs(room:getActiveColors()) do
+		local player = room.player[color]
+		if player and not room:isInBounds(player:getGridPos()) then
+			self:nextRoom(room, player:getGridPos())
+			return
+		end
+	end
+end
+
+function World:nextRoom(room, pos)
+	local exit, entry = nil, {x = pos.x, y = pos.y}
+	if pos.x > gridWidth then exit, entry.x = "right", 1
+	elseif pos.x < 1 then exit, entry.x = "left", gridWidth
+	elseif pos.y > gridHeight then exit, entry.y = "bottom", 1
+	elseif pos.y < 1 then exit, entry.y = "top", gridHeight end
+	local nextName = self:getNeighbor(room.name, exit) or room.name
+	local activeColors = room:getActiveColors()
+	room:save()
+	room.activeColors = {red = false, green = false, blue = false}
+	currentRoom = self.levels[nextName]
+	for _, color in ipairs(activeColors) do
+		if currentRoom.player[color] then
+			currentRoom.player[color].x, currentRoom.player[color].y = entry.x, entry.y
+		else
+			currentRoom:registerPlayer(Player:new(entry.x, entry.y, color), color)
+		end
+		currentRoom:setActiveColor(color, true)
+	end
+	self:playerJoinCheck(currentRoom)
+	self:setLastRoom(currentRoom.name)
+	setEditorRoomName(currentRoom.name)
+end
+
+function World:moveGroup(group, direction)
+	local changed = {}
+	for proxy in pairs(group) do
+		changed[proxy.room] = true
+		for _, cell in ipairs(proxy:getOccupiedCells()) do
+			local room = self:roomCell(cell)
+			if room then changed[room] = true end
+		end
+		for _, cell in ipairs(proxy:getOccupiedCells(direction)) do
+			local room = self:roomCell(cell)
+			if room then changed[room] = true end
+		end
+		local destination, localCell = self:roomCell({x = proxy.x + direction.x, y = proxy.y + direction.y})
+		if destination then
+			if destination ~= proxy.room then
+				for i = #proxy.room.collideables[proxy.color], 1, -1 do
+					if proxy.room.collideables[proxy.color][i] == proxy.entity then table.remove(proxy.room.collideables[proxy.color], i); break end
+				end
+				table.insert(destination.collideables[proxy.color], proxy.entity)
+				self.entityRooms[proxy.entity] = destination
+				changed[proxy.room], changed[destination] = true, true
+			end
+			proxy.entity.x, proxy.entity.y = localCell.x, localCell.y
+		end
+	end
+	self:rebuildCollision()
+	for room in pairs(changed) do room:resolveSwitchesAndDoors() end
+	self:rebuildCollision()
+	for room in pairs(changed) do if room ~= currentRoom then room:save() end end
+end
+
+function World:drawOverlaps(room, color)
+	local location = self:roomLocation(room.name)
+	if not location then return end
+	local drawn = {}
+	for y = 1, gridHeight do
+		local row = self.collidableMatrices[color][(location.y - 1) * gridHeight + y]
+		if row then
+			for x = 1, gridWidth do
+				local entity = row[(location.x - 1) * gridWidth + x]
+				local owner = entity and self.entityRooms[entity]
+				if entity and owner ~= room and not drawn[entity] then
+					drawn[entity] = true
+					local ownerLocation = self:roomLocation(owner.name)
+					love.graphics.push()
+					love.graphics.translate((ownerLocation.x - location.x) * width, (ownerLocation.y - location.y) * height)
+					entity:draw(color)
+					love.graphics.pop()
+				end
+			end
+		end
+	end
+end
+
 function World:getCellFromMousePos(mouseX, mouseY)
 	return math.floor(mouseX / (width / self.width)) + 1, math.floor(mouseY / (height / self.height)) + 1
 end