An army brought to its doorstep. The surrounding fields and villages have been burned. Terms are offered and denied. Trenches and tunnels are dug, and great engines of destruction are wrought. And then it simply. Never. Ends.
No assault is brought, and no surrender is given. The attackers settle in for the long haul. The Defenders' position is unassailable, they must be waited out. That victory never comes, but the siege cannot end.
Crops are resewn. The soldiers settle in. Great walls are built surrounding the Defenders' holding. Towers constructed to oversee every inch of no man's land and outposts to watch the deep caverns. Patrols become formalities. A long fatigue sets in, beset by a yearning for home.
So home is brought to them. Supply lines become formal avenues of trade. The campaign community becomes a domestic community. The soldiers settle. They bring their families and raise their children between duty shifts overlooking that horrible castle. The Defenders could make a move any day now, out of desperation... But will they? Probably not.
Sutlers become merchants. Soldiers into farmers. Prostitutes become wives. Landless knights local landlords, and engineers craftsmen. The Siege continues.
So long, in fact, that it is forgotten that terms may still be offered. Where there was an army there is now a militia. Where once were patrols there are ritualistic 'parades' and grand religious postures in the dark castle's sight. The tools of the army become relics, and their duties become sacred covenant.
And during these long years, mysterious things are happening. An elder dies mysteriously, a well-read scholar's house burns down along with all their books, people whisper that their loved ones have been acting strange, and always are their accusations cast ruefully upon the dark castle the city surrounds.
The people begin to leave. The dynasty who began this cathedral of war has long subsumed. Little memory remains as to why or how any of this city exists, and what is there is veiled in legend and archeology. It all begins to collapse in slow motion, like a glacier receding forever: an earthquake, a neighboring polity destroyed, a great fire, civil strife.
Now, after twenty generations, there are but 500 people left. And there is just but exactly one old, tired, stubborn man who clings to the traditions of his grandfather's grandfather. He stands upon the watchtowers. He parades around the city alone in his faded clothes and tattered banner. He crankily complains that the city was great and beautiful in his grandfather's time - when the people were virtuous and prospered, and the walls were well-maintained - although, in reality even then the City was but a fading form.
One grumpy cantankerous elder, being all that stands between the world and these Defenders of the ancient order. When he dies, in two weeks, and the patrols finally cease, the dark castle will begin to wake.
Its tireless sentinels of the dark castle will see that resistance has dissipated, and they will wake their lord from his millennial slumber.