Forts are usually constructed from the salvaged pieces of a disassembled desk; namely, wood paneling and cinder blocks. In at least one instance a fort was constructed out of couch cushions. They are generally just big enough for one inhabitant of a specific size to fit inside comfortably.
Problem Sleuth, Ace Dick, and Mobster Kingpin all receive commands to build forts during the game. Pickle Inspector, possibly due to his naturally heightened state of imagination has already constructed a fort by the time he is introduced.
PI keeps his TEA SET inside his fort.
Forts also made a small appearance in Homestuck. Dave Strider decided to build a fort out of turntables and cinderblocks, which he then used to reach the cord to the hatch. John Egbert also made one out of cruxite. The Wayward Vagabond also made a fort, which bears the most resemblance to the original Problem Sleuth forts.