Greystone keep and watercourses

Western European style medieval keep replete with Fantasy traps, overlays to simulate flooding and a complex series of water conduits in one of four conditions (dry, still, flowing and stagnant). 

In the original conception the lord of a fantasy city decided it was impossible to keep local denizens from preying on city inhabitants.  So, he created a series of halls below the city, with water supply, to tempt them in to that particular location.  The system is designed to cause the denizens to fight over the water via a series of weir gates and thus control each other.  If one group appears to be dominating then every few years the city rulers cut the water off, leaves it a week or two and then sends adventurers down there to mop up the survivors.