Well, we know that the last time dragons awoke, there were 5 races fighting against them- Jotun, Dwarves, Seers, Mursaat and Forgotten. What if the Eye of the North was built by those races to fight against Jormag? Kind of like Fort Trinity, a combination of said races' architectural styles.
This has actually been my line of thought for a long while. The issue is that the Eye of the North doesn't really fit jotun, dwarf, or forgotten architecture, nor does it fit known mursaat structures from the Ring of Fire. This leaves seers of those five races, however I suspect that they're the creators of those crystalline/ice structures seen in the Far Shiverpeak dungeons (there are statues which are fairly seer-looking in three such dungeons, and those crystalline structures hold an etheric face which could be made to be a seer's or a jotun's).
However, it should be noted that those five races are the sentient survivors
, not just those who fought against the Elder Dragons. We know of at least one other race from that time period: the Giganticus Lupicus. Though they're only outright tied to the Crystal Desert at the moment (and that's just theoretical on NPCs' standpoint).
Just for clarity, the crystalline structures I refer to are these:
And the statues I refer to:
They are associated with White Mantle and Mursaat. The isles were said to be inhabited by people gifted with true sight and the place from where Saul D'Alessio got the Eye of Janthir. They have nothing to do with Seers.
And it may have nothing to do with mursaat. We don't know who inhabited it, per say.
I'm inclined to think that the Eye, judging purely from its appearance, is a primarily Mursaat creation. The rough geometric style shares quite a bit with what we see in the tarnished coast, other unconfirmed Mursaat ruins.
Both the Eye of the North and the Tarnished Coast ruins do not match known mursaat structures. At all.
And furthermore, the mursaat haven't been around since 10,000 BE until recently before Saul found them, so any structures they had made themselves before returning would likely have been long gone (unlike the other races' since they were around to maintain them within the past 11,000 years, though many would definitely be in ruin).
When one looks at all the evidence to whether or not the TC ruins are mursaat in nature, there's actually fairly little strong evidence for such a claim, given 3 facts:
1) We don't know which "forest"/"jungle" Saul traveled to.
2) As said, the TC ruins don't match the only confirmed mursaat structure style, seen primarily on the Ring of Fire.
3) We don't know if the city which Saul discovered was mursaat in nature in the first place, given the new knowledge on them having recently returned (knowledge coming from Arah explorable). Though how recent "recently" is would be rather questionable in of itself.