world.tmx. You should see two layers : one for "objects" and one for "tiles".
Tiles are straight-forward : they're just what they look like. If you place a wall, it'll be a solid wall. If you place a crate, it'll be a crate you can push around.
As for objects, there are two types :
The game doesn't feel right when the camera is always centered on the player. Having different rooms allows the player to focus on the current puzzle, without any distractions.
Room objects have a "Type" property which must be set to "room". You also need to fill the "Name" property with what fits best.
In some cases, doors are shared between multiple rooms. This is why doors are tile objects : by using the name from their "room" property, the game knows which door to open when all buttons in the room are pressed.
Don't forget to enable grid snapping for objects in View > Snapping > Snap to Grid.