The Fortress of Tooth Decay is a large castle that the monster Tooth Decay lives in. It appeared in the Season Fifteen episode "Royal Pudding".
Background
Tooth Decay holds the Princess of Canada as a prisoner here after he kidnaps her during the Canadian Royal Wedding. Later on, Ike Broflovski, Scott the Dick, Ugly Bob, and a few Inuits find the Princess, with Ike using Ugly Bob's ugliness to turn Tooth Decay into stone and save her.