Master of the Monster Lair is a Nintendo DS video game by Atlus and Global A Ent. It's a unique dungeon crawler that requires the player to actually construct the dungeons in order to lure the monsters who are crashing the cities. Once inside, the endless stream of monsters can be defeated and the items that are retrieved from them can be sold. This was another tactic that the government of the city proposed - it would greatly increase their economy, which is also evident by the lack of supplies early on in the shops due to everyone buying weapons and armor to thwart the common enemy.
In the game, South Arc, a city, is being overrun by various monsters. The government decides to build dungeons for them so that they can live somewhere else, though their plan isn't as noble as it may sound, as after they move, the player will be allowed, and required, to go through the dungeons, slaying the monsters as they come. Defeating the monsters will reward the player with money, items, and even food ingredients. Often times villagers will reward you with valuables as well when you perform a certain deed that they request.