The other secret in this area is through the door as you enter the garrison. Notice the locked gate on the far side? Head right from it and jump through a destroyed window onto a small path on the opposite side. From here look at the wall where the locked gate led to. You can use the Aard Sign to blow up a hole that you can jump through (watch the ledge!) From here, you'll find some containers and a corpse with loot, along with a key to unlock the gate back to the garrison.