The Gate of the Feral Gods by Matt Dinniman

A floating fortress occupied by warrior gnomes. A castle made of sand. A derelict submarine guarded by malfunctioning machines. A haunted crypt surrounded by lethal traps. It was supposed to be easy. One bubble. Four castles. Fifteen days. Capture each one, and the stairwell is unlocked. Here’s the thing. It’s never easy. Carl and his team can’t go it alone. Not this time. They must rely on the help of the low-level, I-can’t-believe-these-idiots-are-still-alive crawlers trapped in the bubble with them. But can they be trusted? Welcome, Crawler. Welcome to the fifth floor of the dungeon…