Head down all the staircases, out of the building and toward the bridge, killing the Retainer sitting next to a post along the way. Walk down onto the bridge and into the building to your right, which is now accessible thanks to the thaw. Take the elevator down.
Exit the elevator shaft, turn right, and you'll see three Frozen Porcupines. The long spikes on their back will hurt you badly if you attack them up close and if they roll at you. Use a projectile weapon if you've got one, a shield and the Winged Spear + 7 that you picked up by the three Frozen Dogs as you made your way to the second bonfire, or take them down in quick succession to avoid getting rolled to death.
Pillage the two corpses they were surrounding for Goldenfruit Balm x 2 and Monastery Charm x 2.
This area is filled with enemies who have generally powerful attacks but relatively low HP. It's also full of pillars. Use the geography to your advantage, hiding behind them when you need to rest or heal.
Turn around and head through the arched doorway. A Frozen Porcupine is waiting for you between the pillars along the left wall. Take it down and, behind a pillar in the same area close to the opposite wall, pillage a corpse for Twilight Herb x 2.
Continue through the pillared area to a clearing where you'll fight another Frozen Porcupine.
Continue forward through the right archway, and turn right into an alcove with yet another Frozen Porcupine. Kill it and pillage a corpse there for Old Growth Balm x 2.
At about this point, you'll be informed that your world has been invaded by Dark Spirit Hexer Nicolai, a red spirit who flings several powerful spells your way. He'll inhabit the area, and it's best to let him come to you. When he does, remember that he's highly susceptible to melee attacks, so hide or block when he throws lightning or Dark and wail away when he's not. Also, don't charge into uncharted territory during the fight. Stay back where you've killed enemies. It's safer there.
Head out of the alcove, turn right and, using the wall now on your left as a guide, across the clearing and underneath the leftmost archway. Turn left and kill another Frozen Porcupine. Pillage the corpse there for Amber Herb x 5.
At this point, any number of enemies may be waiting for you down here, including a final Frozen Porcupine and a few spell-casting Witchtrees, who inhabit the area above you but aren't opposed to jumping down from time to time. Defeat them all, using the pillars for cover.
Turn around, ignore the locked door on your left, and follow the left wall to the ladder and climb up.
Defeat the Witchtree waiting for you at the top, turn right and open the chest to collect Boltstone x 3.
Face away from the chest and, using the wall on your left as your guide, head down the path, ignoring the doorway, across a bridge and turn left, then right again until you see a corpse in the distance. Pillage it to collect Magic Stone x 2.
Turn around. See that door in front of you? Walk into the room there and light the Lower Garrison bonfire.