RPG Based

ok, i got my engine, nicely laid out. i got map loading, object etc. ok, my ? is, how to do this. player is walking in a castle....he finds a tunnel, and i want it to switch to that "map" type. i am thinkin bout doing it with bsps somehow, saying, if player is so and so location, then he is in this area of the map. how do the pros do it ? thakns much sorry for the bad english, finals are killing me and i am trying to stay sain with gaming Code makes the world go round...