Hello,
Interesting and enjoyable discussion!
I'll mention two novels that have extensive dungeon-esque crawls and they're definitely as old-school as it gets:
The Man of Gold and Flamesong both by M.A.R Barker the creator of Tekumel, the Empire of the Petal Throne.
I will admit that the Man of Gold is a bit difficult at times to go through unless you are either (a) a true Tekumel fan, (b) have an interest in linguists or (c) fascinated by a unique and alien world setting.
The dungeon part of the first novel that sticks in my mind is the escape through the undercity of Purdimal, a very ancient city. What the Hero (a priest of the knowledge god) encounters there is wonderfully brilliant in it otherworldly distinctiveness (not to mention kind of terrifying).
Flamesong is a faster and easier read as it is written more as an adventure with some excellent world building rather than the other way around.
Here, if memory serves, a group of soldiers stumble across an ancient tube car (think subway) and are propelled half way around the world into the unknown. Along the way they have some interesting encounters exploring the ruins where they have stopped (including a exciting run in with the deadly Ssu)!
Frankly, if you've never had the pleasure of discovering the unique milieu that is Tekumel, you're missing out. Track down a used copy of either book and spend an afternoon (or four) in the most exotic, alien and unusual fantasy world ever created.
Good reading!
Best,