Soldier's Peak gets added to your map once you agree to help Levi Dryden reclaim it at camp.
When you enter the main courtyard of the fortress, you'll be ambushed by waves of undead. They're not too tough. Once the dead stay dead, look for a statue at the south-west corner of the fort's walls. Examining the statue, 4:15, triggers the Ancient History quest.
Head into the Peak.
In the Peak's first room, you'll see a flashback due to the spirits roaming the area. Afterwards, in the next room, you'll be attacked by 2 Rage Demons and an Arcane Horror; be careful.
8:30, in the northern room of this floor, you'll have another battle against either undead or Rage Demons (depends on your combat level). Afterwards, you can interact with the Archivist's Book for another flashback. Also in this room, there's another book, which will advance the Ancient History quest upon being examined.
2nd floor. 12:00
You will witness an old battle between the Peak Wardens and King Arland's forces. Afterwards, a battle will start against a Rage abomination. The abomination will be healed by the zombies that spawn, so kill them before focusing fire on the abomination. If your level is high enough, Rage Demon backup will eventually join the fight. Be careful at this point since the demons can cast Fireball and Fire Bolt.
When the fight is over, head into the next room and examine the jar by the statue to once more step Ancient History forward.
Adjacent to this room, you'll meet a demon.
The demon will ask that you help it kill its enemy so that it can repair the Veil and have its freedom. If you agree to help it and pass a persuade check, you can also get her to reveal the location of a chest containing 8 gold.
But considering its a demon, you can also just kill it and find another way to repair the veil. Killing the possessed Sophia gains you the Warden Commander armor.
After agreeing to help the demon or killing it, the force-field around the door to the next tower will disappear. Continue to the next tower; watch out for the traps and undead on the tower bridge.
In the next tower, examine the jar on the table. If you drink the potion, you'll gain new talents that utilize the Taint in your character's blood.
20:32, in the next chamber, you'll meet Avernus. If you agreed to help Sophia, you can kill him, but otherwise, he'll ask you to wait until he has repaired the Veil before you take any action. While talking with him, you can persuade him that he messed up by experimenting on his fellow Wardens and by researching demonology. This leads him to decide that you, as a fellow Warden, should decided his fate after the Veil is repaired.
With it agreed that the Veil will be repaired (or you deciding to double-cross Sophia), Avernus will head to the Summoning Circles in the previous building. Before following him, examine a corpse on the wall for the second to last step of Ancient History, 26:58.
At the Summoning Circles, waves of demons will attack as Avernus dispels each circle. This fight should actually be easier than the earlier Rage abomination fight since you get brief breaks between waves of attack. When the last demons have been repelled, it is time to decide what to do with Avernus.
You can kill him to obtain his Robes (which make Blood Magic more effective), or you can ask him to ethically research ways to help the Wardens. If you agree to let him live, you'll be able to find a stat-boosting potion from Avernus in Dragon Age 2.
The Peak is now secure! You now have a place to store your old stuff, and Mikhail Dryden (Levi's Cousin) has a decent smith shop. You can sell Mikhail your gear and return later to buyback an upgraded version, and Mikhail can also forge for you Starfang if your find the star metal in a random world map encounter.