So, there's this keep. Long lost, by the forces of Law.
For decades, heroes, lords and other assorted riff-raff have tried to retake the stronghold and its surroundings, for the glory of King and Country.
Ok, mostly for themselves.
Recently, Sir/Lady NPC has managed to claim the keep, clear the area immediately surrounding it and establish a very tenuous hold on his/her new demesne.
There's no thriving little community, here. Just a few surviving men-at-arms, a Castellan, a minor Cleric, a few acolytes, some engineer/builder types, some camp-followers and non-combatant hirelings. They're surrounded. Out-numbered. Running out of time and resources.
All day long, they labor to shore up the gates, patch the walls, fortifying the keep, as best they may.
All night long, they're hard pressed, keeping the forces of Chaos from breaking into the keep.
And the Lord/Lady? Mysteriously incapacitated. Hidden away, deep within the stronghold. Only the Cleric has seen the ruler, recently. The defense of the keep is in the hands of the Castellan. He's like, 3rd level.
Worse, the baleful abominations of the wilderness, seem far too organized.
And not far away...
Not far away, the soldiers have discovered something. They think it's a complex of caves, harboring creatures of Chaos.
They're wrong. It's a fucking megadungeon.
In the face of all this, the forces of the keep must arrange for supplies, reinforcements and, gods help them, maybe some adventurers, to delve into those caves and root out the enemies of the keep.