Arknights is a free-to-play tactical RPG/tower defense mobile game developed by Chinese developer Hypergryph. The core gameplay is that of a tower defense game, with a number of characters ("operators") as towers. Players must place operators on the correct tiles to prevent the enemy from infiltrating the player's base. Because there is often a limited number of viable solutions, especially at high difficulties, Arknights has also been described as a puzzle game. (wikipedia.org)