a fun room.
In general, these are called floating shelves that have an interior structure which is fastened to the wall.
One clue that there could be an interior structure is that the shelf is fairly thick.
If you search on how to install a floating shelf, you'll find a number of methods. Here is
boys room - painted mural on closet door
Great shelf to do in Albert's room!
shelf goes around room - will it be too close to ceiling for trohpies?