When you leave the crypts you may be confronted with a choice depending on the choices you have made, you can go and tell about your discoveries to the mage as he asked you to do or you could relay this information to the guard captain. This decision will carry substantial weight through the rest of the game so chose wisely if you can...
The monument with the key is located east of the great lake, and get ready for another tough fight!
Once night falls, you should prepare yourself again and head back from whence you came and head to the castle,
you will be able to enter it now. To do so, use the key that you newly acquired on the broken monument near the castle's walls.
Descend into the well and enter the castle!