[Theory] Who build the Eye of the North?

I think the Jotun did it.

One of the Norn storylines tell you about a glorious past of Jotun Kings. During the Jotun Path in the Arah explorable dungeon you find an observatory and discover they were stargazers once. Also, the inhuman proportions of it.

Oh how far have the Jotun fallen. They used to be a race of powerful sorcerers and scholars. They were not as floaty as the Mursaat or the Seers, but I suspect their magical aptitude was greater than that of the dwarves. The forgotten, who at the time were well known in Tyria were the fifth major race of the previous cycle.My theory is, that their society was built on magic, where an individual's control over arcane powers dictated its social status.It is claimed that the Jotun giant-kings turned on each other in lust for power, but we know, that the bloodstones were used to put the elder dragons back to sleep, by lowering the ambient magic level to the point, where spellcasters became powerless.The Jotun then may have experienced a civil war over the last bits of magical items, but without any way to create new magical stuff, their society ran out of steam and atomized.

Thrulnn the Lost in Hoelbrak has some stories to tell of the age of giants, when the Jotun and Norn were favoured and the dominant races of their cycle. Apparently the gods were active at that time and once humanity arrived, the magic ceased.

Thing is Eye of the North doesn't match any structure seen anywhere, certainly not jotun which look like they were carved out of singular blocks of stones and left with an egg-like shape split in half.

A little known fact about the Eye of the North, however, comes from the 2007 PCGamer magazine that came with The Movement of the World and The Ecology of the Charr. There was an article talking about landmarks in Eye of the North, most of which is just repeated stuff from dungeon books or in-game dialogue, but there was a tad bit here and there new, one of which was this line on the Eye of the North (page 81 for those with it still):

The Eye of the North looms over the land, an imposing structure of indeterminate origin. It is not human, Dwarven, or Asuran in construction, although it resembles the architecture of Arah and other godly sites. Norn who settled the Far Shiverpeaks first "discovered" the Eye. They sensed something unnatural about its construction, and assiduously avoided it. In recent years, the Ebon Vanguard (a detachment of Ascalonian soldiers, bolstered by refugees and exiles from the Charr invasion) made this area its base and became the protector of the structure. The Eye is built on a perpetually frozen alpine lake. A single, easily defended entrance leads to a wide circular chamber. To the rear of the Eye is a separate chamber dominated by a large pool. The Vanguard suspect the pool to be a scrying device, but it remains dormant. It seems to be waiting for someone in particular...

The three italicized bits are of interest to me. To go in reverse order:

• The Eye is surrounded by trees. This means that the area must warm up enough for plantlife to thrive. This, in turn, would suggest the lake should thaw even if only a little bit every year. But it's perpetually frozen, which to me suggests that the Eye has magic that's keeping it frozen while allowing nearby plantlife. Especially since Gwen managed to make a garden that contains flowers adapted to survive in Ascalon, not the cold of the Shiverpeaks.
• The Eye being unnaturally constructed wouldn't seem too odd, given its magical, but the norn are no stranger to magic or constructing gigantic architecture (even before Hoelbrak). So for the norn to avoid it because there was "something unnatural about its construction" would imply more than normal magic. It would mean magic the norn aren't accustomed to. Which goes even further because in the dungeon Raven's Point the asura Xapp says this about the Raven Shrine: "How utterly fascinating! Those enormous savages have fabricated something quite remarkable. Could this structure, perchance, be drawing the attention and ire of the Destroyers?" This both aludes to our later confirmed "dragon minions eat magic" (Raven Shrine being a magical structure), but more importantly, it shows that the norn are capable of magical construction to the point that asura of GW1 considered such remarkable. That puts the Eye on a whole new level of "magical construction".
• Now most importantly, the comparison of the Eye to Arah and "other godly sites". For reference, the only other known godly sites in the world of Tyria would be the Hall of Ascension - or to show GW2's variation of it. The Hall of Ascension, like the Hall of Heroes, has pretty strong Tarir/Kesho vibes to it; but doesn't really seem similar to the Eye. Arah, in the end, looks nothing like the Eye of the North. It is possible that Arah's original designs back in 2007 were to be more eagle motifed and all that, making it an obvious relation, until they decided to go with our many circles atop of circles within circles designs we get throughout Orr now. However, there is a relation between the Hall of Ascension and Arah still: the Forgotten. It's unclear if the Forgotten made either, but they were faithful servants of the gods, and they had guarded the Hall of Ascension and built many places that is structurally similar to the Hall of Ascension. Similarly, they were in Arah before the Six Gods. They're also capable of magic otherwise unheard of on Tyria.

Looking at everything directly, the culprit would point to "maybe it was the Forgotten!" But to quote Taimi from Season 3:

Taimi: There was a time, way back, when the Seers and the mursaat shared a lot of info. Why do you think they're all so...floaty?

This would have no doubt expanded to the Forgotten, given they apparently fought side-by-side with the mursaat against Zhaitan, and the Enchanted Armor have an unerringly similar appearance to mursaat armor (more so in GW1 than in GW2). And likely to the other two races, but less so since we see that dwarves and jotun were less magically adept.

So when we combine everything, our points are:

• Not built by human, dwarf, norn, or asura.
• Does not match any known architecture.
• Built by unnatural means to norn, who were at the time magically adept enough to impress asura. Likely with strong magic altering the surroundings.
• Related to "godly sites" that tend to be related to Forgotten directly or indirectly.
• Forgotten had shared magical knowledge with Seers and mursaat. Possibly jotun and dwarven too.
• Of Forgotten, mursaat, jotun, dwarven, and Seers, only Seer architecture is unknown.

This leads me to think "The Seers Did it!"

And to support that conclusion, I offer this final piece of interview from Ree Soesbee:

There have been a lot of questions about the Seers and their ancient war with the mursaat. Sadly, these aren't questions we can answer at great length without giving away some very deep secrets of the Guild Wars world. I can say that they are one of the oldest races of Tyria, dating back long before the Gods brought the humans to the world; to the time of the writing of the Tome of Rubicon. In those ancient days, the Seers fought against the mursaat, but they were in turn defeated by their enemies, and their civilization lost to the ravages of time.Some remnants of that civilization, and that ancient time, still remain in modern-day Tyria (both in GW and GW2), but often those who discover such things do not realize what they have found. There are opportunities in Guild Wars 2 to uncover some of the most ancient lore, including previously untold tidbits about the Seers and their story, but such information will be very difficult to come by.https://www.guildmag.com/magazine/issue1-may.pdf

Now some of that is obviously referrencing the Bloodstone's true origins, but at the time, the Bloodstones were not part of GW2 directly so that makes me wonder about the "some remnants of that civilization remain in GW2" and on top of that, in GW2, we know what has been found - Seers being the creators is known without actually delving into Arah (for Tyrians). But if the Eye were of Seer origin, then no one knows, either Tyrian or players, and it was part of core GW2, so it would fit that description better than the Bloodstones.