Why do people climb Everest? Because it is there. Why do cats climb into and onto everything they can find, especially if it is in a dark hidden place or high up? Because they like to lie in wait for you and also to be up high. How she got there? Cats can jump a long way across and/or up!
*looks for old userpic of Smudge on the curtain rail*
*finds pic on flickr instead*