Ithaca, New York, USA
We recently moved into an apartment which has a very tall ceiling in the main bedroom. It was made by removing the original ceiling and opening up an area which was previously part of the attic. There are some platforms and beams up there, and since we couldn't do anything with the space ourselves we made a ladder for the cats to go up there. The design is a very simple one from while pine based on some that I saw in your blog. One of the pictures shows what happens when they want to pass each other on the ladder.
Our apartment is well above ground level, and since we couldn't install a cat door in the main door, we built another ladder for them to go out of one of the windows and down to the outside stairs. There is also a platform below it since the whole contraption is about fifteen feet above ground. I was worried at first that the steps overlapped too much and that it would be hard for them to climb up, but they don't seem to have a problem with it. The design looks a bit strange -- I wanted to avoid driving any screws or nails into the outside of the house, so it's not actually attached at all, just built into place.
Thanks for running such an interesting blog. Cheers,