My apologies to anyone whose eyes glaze over when I get all gaming nerdy. But I gotta get this one off of my chest.
Ideally done with two different groups of players. One group: the adventurers, other group: the monsters.
The monster group gets a dungeon layout, randomly generated, or drawn, or whatever. They get to lay out what goes where (Which is what I did in my previous Old School game, printed off a randomly generated map, and then figured out what the rooms where for myself, so we had a temple, and dormitories, and garbage room, and kitchen, and so forth). And the monster players get an allowance of some type to setup traps and things. They have a finite monster population in the dungeon, yknow, 40 goblins or whatever. Once setup, the adventurers get a go at it. I'm imagining in a completely different session, not "head to head" really. Anyway, the players move through the dungeon as best they can, dealing with traps and monsters until they are killed off or forced to retreat to rest and rearm. Then, during that lull, the monster players may get an opportunity to reset traps, make plans for how to deal with additional assaults, etc. And we proceed either until one side kills off the other, or until some scoring system declares a victory for one side.
Yknow, because only in the most arbitrary and static dungeon do the monsters stand around quietly waiting on adventurers to knock on their door and kill them.