I don't know how to dungeon

[jayem]

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Jayem has also laid out a fairly conventional dungeon which will be a lot quicker to build and should play out faster.

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OTOH, the bandit lair is probably a single [d&d] session, almost certainly the party won't rest and if they have to withdraw and come back the GM should consider how the bandit leader responds to the 1st intrusion.

Sounds a fair assessment, I was trying to focus on identifying metaphorical keys and locked doors (I didn't do it perfectly).
Some of building it up would be that the surroundings would be significant and be part of the 'dungeon'
The other aspect would be thinking of it more like collosal cave/adventure and making each encounter a tactical challenge, although that works better the more mundane you are. (Also then failure is more safe)