What kind of castle do you want? I can't get you a Pepakura file, but I sure can give you an already designed paper model. Here's what I have in front of me that I can send:
<ul>[*]Castle Checiny[*]Cinderella's Castle [*]Malbork Castle[*]Neuschwanstein Castle[*]Sleeping Beauty's Castle [*]Zamek Orlik[/list]
If you are unfamiliar with what they look like, a quick Google search will show you. If you want any of these, send me a PM through here and I can email the file(s) to you.