A Minetown (or some other town) minigame based around classic survival/area defense games:

• Takes place in a smaller-than-Minetown square walled area in a large open part of the mines.
• Orcs, with a very high spawn rate and an increasing difficulty modifier, appear around the edges of the map.
• The walls will block the weakest and the unequipped from entering the town proper, but it won’t be good once orcs start showing up with pickaxes and ladders.
• To prevent the orcs from building up a horde, there are some weaker points in the walls, maybe a gate they can try to bash down or an unrepaired breach. These must be defended with manpower and all the traps you can think to lay.
• Orcs grudge everything inside the town, and vice versa; meanwhile, nothing in the town is hostile to you.
• Shopkeepers sell you items from their limited inventory at very cheap prices.
• There are watchmen who can hold back an incursion, but they’ll eventually fail if the player isn’t able to get there in time.