An underground hall in the slums, a place for dark dealings and other things that people want to keep from the king. It is most famous for the fighting pits, where competitors would fight hand to hand until one was unconscious or dead. Bets are placed on the fights. The stone floors have piles of vomit and blood scattered everywhere. The bar serves watered down ale. The Vaults are mainly occupied at night. There are multiple rooms with ragged curtains serving as doors, where prostitutes offer their services.
[Queen of Shadows spoilers]
After Aelin returns from Wendlyn this is one of the first places she visits, and destroys, also sending it bankrupt by leaving open the cash box