You can't just throw a pillow fort together. I know because I build at least one per day. I've got limited materials of dubious structural integrity and high demands for enclosed interior space. My clients are quite insistent about what they want, and I've got to keep them happy to remain employed.
This one, designed by Patrick Di Justo, uses Buckminister Fuller's principle of tensional integrity to create a lightweight but sturdy structure. It's built around brooms that push each other apart, but never touch each other.
Link | Image: Joel Kimmel