My players have commented that it is a very deadly-seeming dungeon, and I think that is probably true. I use the OD&D method of monster stocking, which means that they can encounter monsters from dungeon levels one through four; this means that there are many monsters that are way out of the PCs league. Moreover, I use the # appearing to determine exactly how many appear, both in wandering bands and in stocking the dungeon (also I usually don't roll 'in lair'). This means there are a lot of monsters, sometimes of a much higher dungeon level. In other words, it is a very nasty place to be.
That said, I do have large bands of monsters disperse into nearby empty chambers, as the G series '200 trolls in one room' model doesn't appeal to me, and to mitigate the danger if the players are tactically savvy enough to isolate the enemy. This does lead to an experience of where more rooms are populated by monsters, but the overall encounters are less brutal (unless they manage to warn their allies).
|Against the unbelievable slog of 10+ hr combats...|
So, how does the party survive? Several factors play a part, but I'd say that the two most important ones are reaction tables and monster vs monster tactics. With the first, I fairly religiously use the B/X reaction tables, which lead to a more exploratory game: humanoids are negotiated with or bluffed (not always successfully), vermin are appeased with food or avoided, and often a monster just ignores the party. To my mind this leads to a dungeon that seems more 'alive', both in the sense that monsters are leading their own lives but also that the dungeon is never truly 'cleared'; more critters are left alive than dead.
The second factor is that the home group is a huge fan of classic 'Doom' tactics, wherein they draw two monsters likely to oppose one another together to take out two birds with one stone. This becomes necessary because monster often guard choke points into new areas of the dungeon, and as the players understand the layout of the place they increasingly desire to find out what's behind that blank map space. The best example was when the party, who had been avoiding a group of rust monsters, realized that they could be used against the sentry turrets from Portal which had been blocking them from a full fourth of the first level.
There are other factors at play, such as regular use of the morale and indulging player ingenuity, but from the DM's side what I've learned is that in the design phase you need to give players plenty of options to explore (while keeping valuable areas 'under wraps' with chokepoints) and in running the game it is best to give players plenty of leeway to avoid an encounter.
Anyways, that's my two cents for today.