As for magical site of power...
Maybe a castle? I mention an old king in one of my designs, maybe he left behind a castle we use as a laboratory for our magic? After we overthrew the monarchy for (insert funny reason here) and installed our own new (possibly communist) regime. Or a democracy, or a theocracy. Or maybe the Monarchy is still around. Or maybe we just reunified the old kingdom after it had broken down, or maybe just part of the old kingdom, with the other part being alikai. I have lots of ideas.
I feel like our design needs obvious function and/or history linking it to the thing. In this case, The Deep. Perhaps the Royal Family has long held an obsession with the Deep and its inhabitants, and with the ways through which the gap between them and mortals might be bridged. Not all of the remains within the Royal Catacombs appear wholly mortal, and odd sounds are rumored to be relatives long thought dead, so utterly changed that they no longer walk among the world of the living.
Vampires? Also, all of my yes. Perhaps it was their obsession with the deep, and surviving beyond death, which lead to their kingdom's end. A curse. Or a gateway. Or maybe... Both. Perhaps, their forms are twisted and strange, and each of the surviving royals are living gateways into the deep. They drank deep of it's power, and were changed. As for fortifications, sure, what about haunted castles?
Other thought, how cool would it be to have a flying castle, powered by the essence of the deep?
Edit: Location idea. Tell me if it's too outlandish.
The Elder KeepA strange place, once inhabited by the old royalty of Zehlin. It possesses a connection to the Deep, cultivated over generations, a byproduct of the Obsession of the royalty, with the Deep, and evading death. You see, as his time wore on, one of the kings, told by a seer who drew their power from strange sources, told him his days were numbered, told him that his kingdom would begin to come undone when he went from living to dead. So, the king, fearing death and the end of his kingdom, sought life eternal. The answer to his problem would not be found while he remained conscious, for, after a hunting accident, he was plunged into a coma. Before dying soon after. And so it went, each successive King or Queen furthering the study of the Deep, seeking immortality and power. Eventually a ritual was devised. It would allow one to become a living gateway to the deep, letting them draw up soul dregs to consume, extending life, granting a form of twisted immortality, granting supernatural strength and speed, and so much more. But at a cost, for consuming souldregs will eventually morph man into beast, and will anger the eldritch things that live within the deep. So, two workarounds were devised. First. Through unknown means, they made it so the sun would repulse the power of the deep that came through the gate. Secondly, they, through means unknown, found a way to grant someone the ability to gain the features of a living thing by drinking it's blood. So, through consuming the blood of man, the changes could be... minimized, and so long as they remembered to stay in the sunlight as much as possible, they would be fine. And so the royalty found the answer to their problem. At a cost however, for their obsession had long taken its toll upon the kingdom, draining it's coffers, and it's lands suffering from neglect by the government. However the Royals cared not, for their immortality had been found, their kingdom would be eternal. As time went on, each successive generation of royals, upon coming of age and siring children, would undergo this process. For five generations. But, time takes its toll. One by one, the royalty lost their humanity. The whispers of the deep persuading them to consume more souldregs, to let them in, let the being's power flow through their body. The foolish, consuming the blood of powerful beasts to become stronger. Eventually, man became as monster, and the royals became abominations that would undo the world. And we rose up to put down the royals, no longer human, but monstrous beings that stalked the night, for as their power began to come from the beings of the deep, the sun repulsed them, burned them. So would we be forced to hunt down those who once swore to protect us, using weapons such as the sun spears, or raising armies of the dead to kill them. Each in turn was laid to rest in the catacombs beneath the castle. Sealed. However, even in the deepest parts of the catacombs, from before the royals found their answer to immortality, appear wholly mortal, and odd sounds are rumored to be relatives long thought dead, but never confirmed, so utterly changed that they no longer walk among the world of the living. However, even today, that castle possesses an unnatural connection to the deep. It is disturbingly easy to break the boundary between our world and the deep within it's walls, and soul dregs seems to well up and pool in the old fountain in the courtyard. The whole place is said to be cursed by the peasants. But for our researchers, our communers, our lunatics, our awakeners, everyone who uses magic, it is perfect. Turns out well over 100 years of the royals seeking immortality leaves the place well equipped for magic research and use.