At the end of Raleigh's Walk load your crossbow with your grappling hook.

Turn right look over the battlements, you should see an open windows in the building across from you.

Aim and fire your grappling hook towards the window.

Once across and in through the window, you are now in St Thomas's Tower, turn left and head towards the end of the building.

Head up the stairs and through two doors, this brings you to the room with the fire that you were in earlier.   Exit through the other door and up, along the battlements to the next tower.   You've been here before, so just go through the tower and along the next battlements, into Salt Tower.

Head down and you will be in a round room with display cases.