There are three portals in bunkers close to each other that all lead to this dungeon, but each portal drops you in a different part of the dungeon. Which you use doesn't really matter (you travel in a full circle around the dungeon). The lever doors next to each drop into the dungeon are all opened by handing in the 4 gems to the field generators and using the energy ring. This starts a five minute timer, after which the six doors open (2 by each drop point). This gives you enough time to run back to the dungeon if using the stronghold statue (the doors remain open for 3–5 minutes).