I had to use my Thesaurus and then turn to the internet to get as many A-Z synonyms for dungeon as I could find. Some were a stretch, but X eludes me….

Asylums & Apparitions

Blackholes & Battlecruisers

Crypts & Centipedes

Caverns & Cavemen

Cells & Cellmates

Dungeons & Demi-Liches

Excavations & Ettins

Foundations & Firetoads

Gaols & Gargoyles

Hideouts & Hobgoblins

Igloos & Ice Lizards

Jails & Jackalopes

Keeps & Kobolds

Lockups & Lawbreakers

Mausoleums & Mummies

Nooks & Norkers

Ouiblettes & Owlbears

Prisons & Pirates

Pits & Piercers

Quarantines & Quasits

Racks & Rakshasas

Reformatories & Renegades

Stockades & Scoundrels

Tombs & Troglodytes

Towers & Trolls

Underground & Umber Hulk

Vaults & Vampires

Walls & Wights

X & Xvarts

Yards & Yardarms

Zones & Zombies