There are two(or three, cant remember :P) in the left tower (if you are facing the doorway
from the statues), one is at the top, and you have to go through another passage in that
tower, if that makes sense (im not good at explaining!). There is another one at the top
of the right tower, and there is one lying lear where the keys go in
hope that helped, unless you already found the last key!