What makes a dungeon fun?

Basically, this topic is self-explanatory. What, in your opinion, makes a dungeon fun? What is your favourite dungeon in a video game and why is it your favourite? If it is something which requires more explanation, then expand upon it. (For example, don't just say puzzles, say what type of puzzles and what's involved in solving it).

Example: I like dungeons with boulder puzzles because they reward ingenuity. Whatever, and go!

I like dungeons with many traps and rooms, and also some puzzles.

I like dungeons that has a side boss/es. They usually give good rewards.

I like dungeons that arwe mostly linear, but don't just guide you in one straight line.
Also as far as puzzles concerned, I especialy like the ones where you have to put a lot of things on the right place or in the right stance which makes something happen with awsome special effects.
Finaly, a dungeon for me is never complete without a boss or mini-boss fight at the end.

One of my favorite dungeons would be the "Niflheim underworld" from "Tales of Symphonia".

it doesn't have al my above menciont characteristics, but the great grafics, good music, chalanging gameplay and good boss make it a dungeon I want to play over and over again.