you mean like Unreal or Quake?, just check for your player being in the corridor or lift that leaves the level and then just load the next level and a stack of new enemies to toast (MMMM! toasted enemies
), then just put your player in the start position and start the enemies and the player running with your main loop code, if you have something that is specific to that level then you can run it from a case structure or just do
if level=2 then gosub level_2_extra_monster()
or whatever, it`s realy all down to what you want to do, for a start just make it so that when the level is finished it restarts but it adds another monster like above, that will give you an idea of what you need to do, then you can alter it to load another map and add the new monster before it restarts and so on, easy!.
Mentor.