I am currently inside the Barad Arthir tower but I cannot find a way to get to the settlement. I see it on my radar but it appears to be below (when you're using the tower's upper entrance). I've used the lower entrance too (the room with all the guards) and the settlement isn't in this lower room either. What I am doing wrong?
We generally refer to the structure as the Minas Morgul elevator. The insides of the structure highly depends on what level you enter through the portal. However, it is also a phased instance. Any time you enter into it, you will be required to "clear the area" first. You must defeat every single enemy inside before the people can come and take over. The usual way you get shown how it works is by obtaining the Introduction: Imlad Morgul quest, talk to Damrod at the camp just outside the tower's base, and he gives you the various circle quests.