Alexandra's husband stumbled onto a good fort-building trick:
My husband discovered a great hack when he grabbed a sheet to use for building forts for our two small children. The first sheet he grabbed was a king-sized fitted sheet, so he draped it over an arm chair and end table and it made a great snug "cave." The elastic edges keep the fort from falling down and the kids can easily wander in and out of it without having to hang it back on. The fort is still standing and no one wants to take it down!
Do you have any fort construction tips to share?