Behind one of the see-through doors in the hall with the "what will the mighty moose be?" riddle is another door that leads to the hall with the deadly cups which you have to drink.
Probably your characters are not strong enough yet to bash the doors.
If you have quaatloo coins left, visit the dungeon with the casino (Stronghold Slithering Cult, I believe) and use the coins to boost the strength of your first 2 characters.
If that still is not enough, you might try Wizard's Eye to find the correct door and Teleport or Etherealize to get through. Or visit magic fountains which increase Strength temporarily.
It is also advisable to have resurrect, raise dead and stone to flesh spells ready in the Cathedral of Carnage [the name is NOT Cavern of Carnage].