When you are making a kitty castle, you will likely want to fold carpet over to make a window, or to make make a nice edge on a wall. All you need is white glue and binder clips to hold the carpet while the glue dries.
Pictured here is an opening on the tower of the castle that will be going to Lyle as soon as I finish it. (The area around the opening without binder clips was previously glued. You may remember Lyle from the Lyle cam.)