The Frost Gate (outpost)

Bonus Mission
The bonus mission is triggered by rescuing the dwarf located in the dryder caves. He'll ask you to help him retrieve some secret plans from the Stone Summit dwarves. The secret path he mentions is roughly straight north from the nearby besieged bridge; once you've found that, simply follow the path around to take over the derelict ballista, then go back and head across the bridge. You'll need to carry the plans back but there are no ambushes.

This is a lot easier if you clear out the path to the derelict ballista before talking to the dwarf about the plans, otherwise he seems to invariably die.