I dont know where is that from the inside
Ok, I've figured out what drawbridge you meant - it can simply be lowered from the courtyard where you help Paladin Cork, there's a wheel there.
Also I found another way into the dungeon level of Fort Joy:
4. Go to the elven hideout at the south part of the map - there's a boy with whom you can play hide and seek. After finding him two times he shows you the way to his secret friend (or you can find it yourself with high wits) - a Source Hunter bound by Braccus Rex long time ago. He gives you a quest to find his bound soul and points to a hidden switch on waypoint statue of the Seven gods. This will lead to the dungeon level and repeat of 1.2.
