I think cats like 'forts' as much as kids do! Mine love it when I change my linens and build the pillows up high, and put the sheet over it. It is like king of the moutain on a tumbling mat.

I had an office chair that got used the same way! It also flexed back a little bit. When one cat would climb up the back, it would tilt at an angle, and they were always so startled. Then, when they jumped off, around and around the chair went. I think the inside of a house is like a playground for cats.