Take things out of the box and put them where you want them. It is remembered, so the fort is still standing when you come back.
Everything in the box is drawn rather than pictured. Each thing is a little routine that paints a pillow or a draped blanket or a sleeping cat out of curves, so there is no artwork in this drawer and nothing is loaded to build a fort.
It is placement, not physics. The brief allowed either, and for a fort the fiddly version is worse: things stay exactly where you put them instead of sliding off a cushion the moment you let go. What you do get is a real stacking order — anything you pick up comes to the front, and you can push it behind the rest — which is most of what building a fort out of furniture actually involves.
The blankets hang. A blanket's lower edge is a chain of catenary-ish sags computed across its width rather than a wavy line drawn by hand, so a wide one droops more in the middle, the way a real one slung between two chairs does.
The lamp is a real light: everything in the room is tinted by how far it sits from the lamp, so moving the lamp changes what the fort looks like. Turn it off and the room goes to moonlight.
Your fort is kept in this browser and nowhere else.