Oslo, Norway.
Photo: Eirik Newth.

Eirik wrote about the ladder in the guestbook - here it is:
"The ladder is made in such a way that it can be removed at short notice - we live in an area with strong restrictions on any additions to the buildings. Our cats, Ada and Linus, needed a couple of weeks of training to get the point, but now they are running up and down the ladder with joy. Cat ladders rule! Ada and Linus have their own flickr stream, by the way: http://www.flickr.com/photos/tags/adaoglinus/ :-) "