The Crowbar item is a pretty handy tool. You’ll use this to pry the second rock that you’ll push to the fountain. You’ll also use this to open the closed window in the Armory area. Then use it to open the lock of the door of the castle.
How to Get the Crowbar
When you enter to the castle ground, there’s a tomb with an open doorway here that leads into a mausoleum. Go inside and pick up the book and the crowbar.