Mordor is the correct answer.
I was specifically thinking of Sauron's dark towers, with Beleriand being Tol-in-Gaurhoth; probably should have said 'West Beleriand' to keep people from thinking of Angband.
Back to you,
G55 (and,
Urwen - did you just unflinchingly cast the Halls of Mandos as a dark fortress? I kinda like that!)
hS