Once you're inside the fort proper, there are a few ways to do this, but I recommend going through the entire fort and murdering every last magister for the experience. I started from the bottom up, killing everyone and then exiting out the main gate. Lots of encounters and loot to be had, plus some good conversation.
- Escape through the main room at the top of the fort with High Judge Orivand in it (you can technically sneak by and head down the ladder without a confrontation). Take the high ground immediately, burn down Orivand, and teleport their melee units to the ground below.
- Escape through the main gate by heading outside and confronting a group of magisters arguing with a paladin. He'll support you in combat, making it a cakewalk, especially if you come out from behind him (near the Orivand room). Head east and leave to the Marshes.
- Escape through Flenser's playground at the bottom of the fort. You can exit through the sewer grate located right beside Trice, but take care of Flenser and all his zombies first. Having a strong front line or battlefield control is necessary to avoid the brute force of the zombies.
- Escape with the young boy on a boat in the middle of the fort by finding and saving the tortured magister being questioned by his peers for smuggling out children.