Use nothing but cardboard boxes and packaging tape
Build several rooms and hallways connecting them
Allow for fort to be folded down without destroying structure
Add doors from creased flaps
Measure square footage available, noting pre-made edges and corners
Estimate space needed for rooms of various sizes and of hallways
Draw layout for several rooms and hallways (even if only a few inches long) using available material
Cut boxes down to each side
Construct each room and hallways as designed using packaging tape to hold it together
Leave 1/2" of tape between each edge to allow for maximum folding potential, allowing for structure to be collapsed easily
Use tape or pre-made creases for hinged doors
Cut pieces of thicker cardboard into right triangles about 3" on each leg
Tape only one side of the triangles into the less rigid edges in the upper edge along the roof
Swing triangles perpendicular to taped wall for more support, swing them back to allow structure to collapse
Add Velcro in between rooms to allow for modular building (like legos)
Remove tape holding each room / hallway together
Fold outer edges about 1" to allow room for Velcro strips
Place Velcro strips on new folded edge, making note to keep each side constant to ensure sticking (hooked side on hallways, soft side on rooms)