Sunless is a co-op tower defense mobile game. In this game, the player can either select a stage or create a room to play with friends. Despite killing all enemies, players need to protect their castle. If the HP level of the castle drops to 0, the game is over. In each Attack Wave, enemies will be generated from the enemy flag. Players need to kill all the enemies and proceed to the next wave. There are 9 waves in every stage. A small gang boss will exist in every 3 waves. A final boss will appear in the last wave. Lastly, every player has his / her own mission. He / She needs to complete the tasks to gain rewards and thus upgrade the alliance power.