Your goal is to find and unlock all special (red) chests in the dungeon. You can find these randomly or follow the pointer next to your character. After finding one, in order to unlock it you have to start the encounter and stay inside the circle for enough time while defending yourself from an enemy wave. The wave gets stronger with each red chest.

While exploring the dungeon you will find chests that give small buffs (e.g. increase tower attack speed) or heal you.