There are two switches that when pulled with spawn monsters. After they are all killed you should see two platforms appear that you can get to the 3rd switch and pull that. 3 weapons come down from the celing, choose one (they are all basically junk) and the door will be unlocked.

Every time there I run into trouble on the road, there is always a dwarf at the bottom of it. Don't they know how to drive above ground?