Those structures are in the direction of the inaccessible Western Shroud. We can see more of them in the Central Shroud region by The Standing Corses if you look directly west as well.
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Currently... what quests or npcs can tell me story about ampador keep and their civilication?
Unlocking Amdapor Keep lists a fair bit of information about it in the Duty Finder description, plus the SCH quest line deals with it a bit (apparently Amdapor attacked Nym 15000 years before). But their history is still very enigmatic at this stage.
1. Theres something weird on the ancient Nym-amdapor territorial map though, as we know, Nym have a temple santuary aka WP @outer LN and some fort @ sahagin lair (as indicated by the SCH pre relic location) and a fort / settlement at God's Grip (when Gerolt said the Nym outmanouvered the Amdapor forces who were sieging them),
2. Looking at Amdapor, we know they have a keep at the NE of their Amdapor town but not on their SE borders towards thanalan and not on NW, which means whatever threat they have, its coming from the NE as in Ala Mhigo direction or perhaps Gelmorra who resides in the Black Shroud too. For comparison just look at Ishgard with their W, E, SE, SW fort / stronghold location but strangely enough not on their north (perhaps they have excellent aerial defense?)
3. If you look offmap the Amdapor town, u'll see some weird shape of the map as if something resembled a blight have infected the area and every blight in other games' lore would stem from forbidden magic stuff
I don't know what started the ampador/nym war. But I do know why the empire attacked. Partly it was your basic land grab. Likely similar to what one person said not long after the United States formed. It was manifest destiny. Basically it was our god given destiny for our country to reach from one coast to the next.
And once they saw Midgardsormer at the silver tear falls they decided that Eorzea is riddled with false gods ( the twelve/primals ) and they have the eorzeans living in chaos. So now they have have a mission that the common Garleans can get behind and believe in. The Garleans aren't cruel and greedy invaders. They are SAVING the Eorzean people from these false gods and bringing them true civilization.
Naturally eorzeans didn't quite see it the same way.