Know your enemy
Different dungeons call for different approaches. Learn to identify the patterns and characteristics of the dungeon you’re facing, and use that knowledge to make educated guesses about the kinds of threats you might face. A key question is the “level” of the dungeon you’re facing, and whether that’s greater or less than the average level of your party. More difficult dungeons tend to have commensurately greater rewards, but demand more cautious tactics, especially from lower-level parties.
Dungeons can be grouped into five broad classes. Knowing which class a particular dungeon belongs to can help you anticipate likely dangers, though dungeons are highly variable even within a given class.
Old-school
(Keep on the Borderlands, et. al.)
These are the old classics, and the handful of modern dungeons that emulate them. Old-school dungeons assume large parties (up to 10 characters), and expect frequent casualties. That means large numbers of monstrous humanoids, deadly and often arbitrary traps, and little or no internal logic. On the other hand, treasure in these dungeons is often very generous, and magic items are abundant. Old-school dungeons often get more dangerous as you go deeper into the dungeon.
New-school
(Tomb of the Serpent Kings, Hole in the Oak, )
New-school dungeons often have more literary flair than old-school dungeons. They have more distinctive aesthetics, more internal logic, and often more unique encounters. New-school dungeons tend to have smaller encounters, and less arbitrary traps. They can still be very deadly, but they will more often have “tells” or internal logic which identifies risks early on. Treasure in these dungeons tends to be less generous, but magic items when they are present are often very powerful and unique.
Negadungeon
(DFD, Xanadu)
The defining feature of a negadungeon is that the only winning move is not to play. They’re extremely deadly, often have little or no reward, and can sometimes have campaign-ending consequences. If you’ve realised you’re in a negadungeon, it’s probably already too late.
Another, rarer class of dungeon is the “anti-dungeon”. This is kind of like a negadungeon-lite. It might not be as deadly or as campaign-threatening, but it’s still a waste of time to explore. Best left alone.
Puzzle-box
(Prison of the Hated Pretender)
The puzzle-box dungeon revolves around a single enemy, situation, or magical effect. They present a complex problem which the party must solve, often while also encountering dangerous enemies. The rewards for solving the problem can be great. More than any other kind of dungeon, puzzle-boxes reward careful note-taking. Even quite simple situations can feel very complex when hidden behind the fog of war. This is countered by checking assumptions, reviewing notes, and testing theories.
Megadungeon
Stonehell, Arden Vul, Dwarrowdeep, etc.
Megadungeons are extremely large dungeons, more than a hundred rooms. They can have features of the new-school or old-school dungeon. More than just size, megadungeons are defined by having several “zones”, each with a different theme and a different set of enemies. A common feature of megadungeons is “faction play”, multiple groups of enemies with conflicting motives that might be played off against each other. Another common feature of megadungeons is that each level has more difficulty and danger than the one before it.