I would strongly advise against applying to Castle, as due to its somewhat ancient setting it's not exactly disabled-friendly (e.g. all meals are in the Great Hall, which is only accessible up steps and virtually all social life centres around the Undie, which is down steps (although not completely inaccessible to wheelchair users). Basically it'd be completely impractical for you, in getting around the college. Since they can't modify the buildings in any way, I don't think this is going to change either. If you want to come to Durham, I very strongly recommend that you visit before you apply and check with the academic department that everything will be accessible to you.