Crypt of the Moaning Diamond

Crypt of the Moaning Diamond, by Rosemary Jones, is the third book in the loosely-related The Dungeons series.

"The Siegebreakers are a tight-knit group of eccentric mercenaries who boast they can safely bring down the walls of any fortress, and will do so for the right amount of coin. But when the walls of their latest job crash down on their heads, trapping them in ruins treacherous with magic, monsters, and ever-rising water, it's all they can do to stay alive. Undaunted, the Siegebreakers are determined to escape and finish the job."