McH

Since its an Ancient Fort, I think that the Monsters that should be inside are All the Undead Forms, Skeletons, Zombies, Ghosts, And a Powerfull Lich.

A Wizard can give the Quest to retrieve a Tomb or Idol, at the same time he can give the Party a Ward or Item that will unlock the Enterance.

Also the party can investigate what happened in the Fort, why there are Undeads, what happened to the People who used to live there ( By talking to a Ghost or Two inside the Dungeon )

I agree that the Quest should be Given at chapter 5-6, when the parties are powerfull enough

Thats my opinion Anyway