My kitty has started using the stairs and loft I put up originally for my other kitty over a year ago — and loving it.
Naturally, I had to paint the stairs in a cool color to coordinate with my office. The stairs and loft are by my web client, Kitty Cat Loft. I loved them as soon as I saw them. On cat is very light so bounding up the stairs is easy for her. The other is 11 pounds, which isn't heavy but she is voluptuous and maybe the stairs are too close for comfort. Still, the one using it has been with me for a year and has only just started using this awesome piece of cat furniture. She hangs out on the loft and has appropriated it.

The stairs are extremely space efficient, taking up only 14" of floor space. They fit in any corner or the middle of a wall. The loft sits over any doorway. I think you could put it over a window too.