Start by speaking to Commander Denulth in Burthorpe, and ask about a quest. He will tell you that the trolls have taken up a heavily fortified position on Death Plateau, and he needs help looking for another path to the Plateau.
Head west/northwest through Burthorpe and look for the cave entrance just north on the other side of the ruined wall. Talk to Sabbot the dwarf and ask about another path to the plateau. He won't help you, so just say tell him you won't move until he helps you. Say Fine! a few times, and he'll agree to help. He'll finally suggest to go underground.
FredaHead further west and look for the path onto the mountains. Follow the path to Freda's house and talk to her. Talk to her, and you will finally suggest you do her errands for her while she works on the the information from the survey.
Home teleport back to Burthorpe, then head northeast and talk to Dunstan outside in his forge next to the weapon seller's house. Talk to him about re-spiking them, then return to Freda. She will give you the survey.
Look at the survey and return to Sabbot's cave. Mine the northwestern wall and enter the cavern. Climb down the cliff side, head east and squeeze through the gap, jump the crevice, swing on the rope swings, jump the stepping stones, squeeze through another gap, then finally climb up another cliff side (phew!).
Exit the cavern and you will notice that you can see the Plateau easily and the trolls don't know a thing about... Oh. Talk to The Map, then fight him. Afterwards, home teleport to Burthorpe and talk to Commander Denulth.