There are typically two main problems when it comes to building pillow forts: structural integrity and a lack of building materials. And both of those issues are solved with these wonderful engineered cushions called Squishy Forts, which use super-strong magnets to ensure they stay standing for longer than 10 minutes.
Currently raising funds on Kickstarter, $100 will get you a basic Squishy Forts set that includes 10 cushions held together using rare earth neodymium magnets. But for $$100 (Australian), you can bump that up to the ultimate set -- which comes with 25 cushions, letting you build a living room metropolis.
And when you're done playing, the real genius here is that the cushions stack together and tuck away inside a footrest so they're out of sight. But who would ever want to stop playing with these? [Squishy Forts via ThisIsWhyI'mBroke]