In centuries far past, age fell upon the frame of the Holy Warrior's son, even though he bore the sword imprinted with Her eternal soul. And as he felt the time of his passing coming upon him, he gazed out upon Her keep from its highest tower, and he pondered. He was but a pale shadow of his mother, who had taken up the golden blade proffered by the gods, and driven the minions of the Foul One from this land, he knew, but there was still service he could offer. He had drawn plans for the defense of that blade, and of the seal against the demons, a mountain erected by men, who would live within it. But mere stone alone could not hold forever. Something more was needed.
The eldest of the Hellschwert line descended into the depths of the keep, and knelt then, his knees creaking with augue as he did, and prayed before the Holy Blade. When the words had finished echoing throughout his soul, he adjourned again to that high tower, and called forth the priests and the magi, and ordered them to build, and left them instructions.
For six months they prayed and worked their spells, and as he expired, the old king laid himself down upon the bier that they had made for him before the Bright Blade. And as he did so, a cold flame of pale green leapt from the Blade to his body, and it consumed his body, until it was naught but ashes. And taking up the ashes, his sons and daughters blended it through the mortar that would build the great walls that had begun to take shape. And when they expired, and their sons and daughters, and their sons and daughter, and all the descendants of the Holy Lady were brought forth to the bier, and their ashes were used to build the Keep of the Bright Blade.
Then, after many years came the orcs of the Northern mountains against the Keep, and against the walls themselves, no engine, nor any dark magics could prevail, though they shook the earth and split the sky in their efforts to free their dark masters from the prison in which the Lady had sealed them. And then, at last, after months of siege, through stealth and treachery, they stole into the sacred keep, and threw open the gates thereof. Standing tall in that same tower that his forefather had looked out over the keep from, King Lyle II raised the golden blade, and called out to his ancestors. And from the walls themselves they came, first as filmy phantasms, then at last as glowing figures of themselves, and set they upon the invaders, and smote them down with sacred might, until at last, with many sorely wounded, and yet more expired, the fell beasts fled, until not even traces of them could be found, and nevermore set against the Brightblade keep.
An ancient city and castle, the Brightblade keep is unique in the world for being a man-made mountain, erected over millennia. A maze of rock walls many hundreds of feet high, each wall more hundreds of feet thick, the city itself is carven into those walls, so that nothing exists that is not a part of a defense of the central keep. While that maze is, itself, a trap designed to permit the easy slaughter of entire armies from within the city, it is not the sheer mass of the walls, nor the confounding architecture that is its final defense.
Rather, the bodies of all the descendants of the Holy Lady who founded the keep are mystically burned upon a sacred pyre, be it there in the keep, or at any of the smaller holds they have established through the lands, and those ashes are incorporated in the walls. Any living descendant of the Lady may call their spirits to the defense of the city, so long as they bear her sword, and have been accepted by it. Once they answer that call, the spirits will do their forthmost to destroy the invaders by might and magic, with all the abilities they had in life, augmented by their incorporeal bodies, and the blessings of the Lady.
Furthermore, their presence, and their faith, reinforces the walls far beyond the normal for stone and mortar - truly powerful magic would be needed to breach them without treachery. As well, they suffuse the tunnels throughout the walls with a soft, dim light, and in winter, a quiet warmth, just enough to blunt the edge of the dark and twisted corridors could otherwise bring against the minds of the inhabitants thereof.
to add an idea. It only takes a moment.