1 - Climb up the main tower. After the statues, a bit higher up, you can climb inside through windows. It's inside the tower.
2 - Go all the way up inside the tower until the way is blocked. There's an open window. Exit and climb up to the next open window. Once inside you can go up the stairs onto the center balcony of the tower. Now, at the outside, climb up to the top. It's in the top room.