I'm not too sure the Darkhold was built by House Dzemael... That portcullis at the front? Sure. The rest? Doesn't that all strike you as slightly too Dravanian is style? The round doors especially, where are they used in modern Ishgardian architecture? Nowhere. Then there's the age of the place... And fact that a nasty Voidsent statue was waiting to be woken... Some of the more open sections you can see more bridges going off to other places, and the whole area kind of reminds me of a gloomy, claustrophobic version of Mourn... Then there is the Knight's Quarter... Notice those stone/web like formations? After going through the Aery, you know what that reminds me of? A Dragons nest... A really old, forgotten one, but that's what it looks like... I really suspect The Nail to have been a significant location for Dravanians, similar to Mourn and Sohm Al...

I really don't think Dzemael built the place, I think they rediscovered it. Can't imagine the formation of the modern Holy See was a smooth one, while we know some of the Twelve Knights simply decided to (effectively) retire, would their families willingly give up their position as well? I can picture early "heretics" fighting back against the new Four Houses rule, with the Darkhold perhaps being where some retreated to try and survive, even calling upon Voidsent for help (which might seem unlikely until you realize the Vault had the Holy See summoning Voidsent as well...). In order to rewrite history, Ishgard pretend the place never even existed, a thousand years later House Dzemael rediscovers the caves and decided to repurpose them, right before Batraal wakes up and the Garleans surprise them...

Doesn't really mean Dzemael isn't responsible for most of the construction in Ishgard, but that just further damns the Darkhold as not their own; None of the construction in Ishgard resembles Darkhold at all. The only place that comes close to resembling Darkhold are the ruins in Dravania...