Once you end the mission at the fortress, go out through the gate from which the Hollow Men came  and turn right. There you will find a bunker with a boarded up entrance. Destroy the blockade and pick up the key behind it .
After entering the Ossuary (requires "Gone, but Not Forgotten"), take a look around the crypt on the left . The key is hidden inside one of them .
To get the next one, go up the stairs to the top of the nearby building  and jump down through the gap in the balustrade on the left .
Head to the forest village and turn left before the entrance (behind the drowned platform). Stick to the rocks surrounding the village and you will reach the ruins  behind which the silver key is hidden .