Here's an alternative if you can't let your cat out for whatever reason, and don't have any screened in area for them to enjoy a bit of the outdoors. We built this at our old house and the cats just loved it. It was screened on the sides and top, and the base was painted plywood. An alternate design would be to just make a box about the size of an air conditioner and then you could stick it in the window without any outside supports.
The regular window still worked as usual, and could be closed and locked as always. But on a nice day this was a favorite perch for a cat nap.
This was on the back of the house and invisible to the neighbors so we didn't care how it looked. At our present house we don't have the right spot for one, unfortunately. But we are looking into screening in our front porch this year--for the benefit of people as well as kitties. It'll have to be aluminum screening because of kitty claws.
