There are currently 3 possible entrances to the underdark. So don't worry, plenty of ways to get there.
Thing is, if you go the route I go, you don't have any reason given to go to the Underdark at all. They said that the area was optional anyways in the full release, however, in the meantime if you do what I did and have the quest objective next to travel to the Towers, you're left hanging as the roads there are blocked for the EA.
I'm assuming there is a conclusion proper if you actually go to the Underdark and take the route to the towers there, mind.