I made a couple window ledges and a ladder to get up to one of them. I have a picture of the one with the ladder...
You can't really see how the ledges work, but they have industrial velcro at the top (it's actually really strong, so don't worry about that), and from the side, the top and sides look like this:
The window and wall would be on the right side of that drawing. Edit: My little drawing isn't working... pretend that those diagonals are touching top to bottom to form a large diagonal.
I used pine boards, and cedar 1x2's. I put a bit of molding around the side of the platform, and stained it. The kittens absolutely loved them, and if I hadn't moved so soon, I would have sewn little cushions for the ledges.