Heiankyo Alien is a top-down strategy game with puzzle elements. The player controls a Heian period policeman who must defend the capital from an alien invasion by digging holes in the ground and filling them back in after the alien has fallen inside. The player receives points for each alien caught, and the faster the hole fills after the alien falls, the more points are scored. The number of aliens increases as you progress through the levels, and they can escape from their holes after a certain period of time has passed or if another alien passes over their hole. The player loses if they come into contact with an alien. Each level has a time limit, and the number of aliens increases dramatically when this limit is reached, essentially preventing the player from completing the level.