I love dungeons. Yes I do.
Probably the biggest addition you can make to dungeon design is verticality. So many dungeon maps are flat. It doesn't take much to make a dungeon so much more interesting when you add a bit some verticality. One of the best examples of this is from Lost Mines of Phandelver. The first dungeon - The Cragmaw Gobins Hideout - is absolutely fantastic for this. Multiple paths, and lots and lots of verticality. That encounter with the bridge across the path is just such so much fun.
This model really brings it to life:
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