Hellgate is one among other structures in Ab'Dendriel that has been there for a very long time, even before the elves came there. It is being used to throw in criminals that you want to get rid of. It is secured by a sealed door, but if you bribe Elathriel (sheriff, executioner or avenger - Az'irel in elven tongue - of Ab'Dendriel) with 5,000 GP he will give you the copper Key 3012 which you can use to open the door with.
Beside all of this, Hellgate has more secrets. Between these secrets we can find the Treasure Chamber, a place with items only dreamed of, and a mysterious room guarded by a giant spider. This mysterious room has 3 chests on it, and is sealed with a magic wall.
Hellgate has also the entrance of Draconia. There isn't any Protection Zone in Hellgate. The way to enter Redbone Castle is killing the Demon Skeletons guarding in the entrance.