We made a ladder (completely uninspired by this which pops up occasionally).

This was after moving the cat to an upstairs apt, which had always been an outdoor cat in Vermont.

Took him like two months to figure it out, even after terrifying and pissing him off by stranding him in the middle of it.

He was a thankful and happy cat when he finally realized his freedom.

Ours was uglier than all of the shown above (a spiral staircase around a pvc pipe), but it was a fun project and very rewarding.