The game has 12 levels as of 1.0.15 with the exception of games taking place in the Drill Pod, which is endless. Each level takes place in a different environment (sewer, laboratory, etc...). Each pod/ship is a different experience because each one of them has special rules. For instance, the Infirmary Pod : All passive skills with health regen provide a defense bonus instead.
With the starting escape pod, the player will start with two heroes. Heroes can be unlocked after finding them in the dungeon and keeping them on the player's team for three floors. The max number of heroes on a team is four. Heroes can be used to unlock doors which essentially counts as a turn. Each door unlocked will trigger events and monster spawns from unpowered rooms. Heroes in unpowered rooms will also prevent monsters spawning in those rooms. The goal of the game is to move the crystal from the starting point in the level to the elevator. The player will have to collect dust to power a path from the starting point of the crystal to the next elevator.
Check out the Beginner's guide for more information.