Ahhh, I didn't know that. Makes more sense now!
This is an extremely plausible theory! I always interpreted the large pulsating thing to be his heart (I believe Alisaie called it such, but she could be mistaken).
Exactly! Boulder Downs is unlike anything found in Eorzea. To me, it seems like the structure was unearthed rather then falling from the sky:
1.) The structure isn't mangled or broken (like the Dalamud piece imbedded in The Nail). It's entirely intact with almost no damage at all to it.
2.) The structure is built into a natural depression that could also be seen in 1.23. Even in 1.23, you can see the depression and you can see the rings that show similar patterns to the crystal formation (see screenshots).
3.) The structure looks complete. Heck, there is even an entry way into the base of the structure (albeit closed off see screenshots).
4.) The crystal coloring isn't congruent with other pieces of Dalamud (Over aspected/orange coloring/chaotic crystal) but gives off a dark red hue that seems dull (giving the impression that it's been there for a long LONG time).
5.) There are lights on the structure much like we see on the Allagan Tomestones of Philosophy and Mythology. We know the Tomestones act similar to hard drives and contain information of all kinds related to the Allag civilization (leading to my assumption that the structure is a repository of sorts).
6.) Ah, the Giants. I didn't think of this one. It does seem odd that they are there much like how they roam the area around Crystal Tower (the only confirmed standing Allagan structure). Makes sense that they would also be in Boulder Downs and Crystal Tower as both may be OF Eorzea and not of Dalamud etc.
7.) Dalamud crashing to the ground caused major shifts in climate as well as terrain. I would imagine one of the "swords" could have struck a fault line causing a tectonic shift (earthquake) that caused the structure to become visible.
The Nail:
Boulder Downs (showing the crystal ridges):
Allagan Structure (showing that there is power going to it):
Bottom view of the Allagan Structure:
Top view of the Allagan Structure:
Allagan Structure Entrance (Thanks Razia Sultana for putting the entrance into perspective!):
1.23b Boulder Downs:
From the NOAH report it gives information about Archeron being a great Allagan General and supporter of the people. The Empire however, paints him as a usurper and traitor attempting to cause civil war. As punishment, his mind was re conditioned and his body was fitted with modifications (he was also used as an example to de motivate others with the same ideals. Some fates are worse than death. To be robbed of your free will and serve as a gatekeeper for eternity is a prime example. Can't kill him as it would make him a martyr strengthening peoples resolve against the Emperor. What they did to Archeron is a win/win for the Allag).
Archeron cannot be persuaded or stopped because he doesn't have the capacity to. The Allagan Empire robbed him of that.
From what I understand, Crystal Tower was built to house the tomb of the first Allagan Emperor, Xendu, as the Allagan believed that one day he would awaken and lead his people to glory (much like Egyptian beliefs and mythology. Take a close look around the Labyrinth of The Ancients. You will notice a theme of "eyes". Eyes are everywhere, but are mostly seen in the final area. Egyptians have a God with the Symbol of an eye. Horus. The God of sun, war and protection).