Paint solid rock, then carve rooms out of it. Export packs it into rectangles automatically.
Biome
Tool
Fill rock1
Carve (empty)2
Thin ledge3
Danger4
Spawn5
Checkpoint6
Door7
Select8
Brush
World
Levels (dev server)
Export
Start room
Add room
Map file (dev server)
Set the ★ start room above · drag a room to move it (snaps to grid) · click two doors to link/unlink · right-click removes a room · Save ▸ writes the map. (Doors are placed in Room mode.)