Deep Cathedral

This large place of worship seems perfect for a party of heroes to barge into just in time to stop a dastardly ritual, and it's got the space and the cover to make it a time and a half! Just a handful of archers or crossbowmen on the balconies and the cover of the altar and low wall at the end make it more than a challenge to get to the big bad and stop whatever happens.

By the same token, imagine the kind of epic last stand this space could provide! Hordes of infernal creatures swarming around, plucking hapless defenders off balconies and dropping them to the stones below! Fighting pew by pew until you're backed up against the altar as the cleric chants and prays for divine intervention! THE MIND BOGGLES!