So, that big wall in zone 5 of DT...

[Minali]

Yes, the WALL one, in Heritage Found.

What's its purpose? I skim read through transcripts in the usual places of some main and side quests in this area, checked the Sightseeing Vista descriptions of this map and searched online, but I couldn't find a clear answer.

We know it wasn't on the Source "before", it's made out of a ton of electrope after all (and Erenville says as much on arrival).

I can guess that it would technically make sense to have a wall inbetween the area where people live and a lake made of ruins and lightning explosions. Would be kind of dangerous for kids to play there.

It seems electrope is or was being mined somewhere in the general area. Not sure if they brought the mines with them when they merged or if it's a regrowing resource or whatever though, but either way the wall could contain methods of transportation of the raw product. Or it collects/transport lightning energy, like a lightning conductor but better, and transports that instead. Or both.

(If you wouldn't be able to walk around the wall and people would be living in the place where you can do this, I could imagine Sphene and Calyx wanted to keep scavengers and prying eyes out of old Alexandria, but, well, you can just walk past it, so...)

However, this is all a mix of reasonable assumptions or pure speculation on my end. Anyone here who might know an actual reply, maybe from Q&A on a convention or an interview or so? Or anything in game I might have overlooked, some item description or so?

(It's not a super important question, I know, I'm mostly requiring it for my WoL's head canon and wondering how urgent it would have been to build it. However, I'm also simply intrigued how such a massive wall is just standing there but no NPC bothers talking about it, nor is there a Sightseeing Vista. But it's also not like I'd be busy talking about some basic local infrastructure irl all day unless anyone specifically asks or anything breaks...)

[Ilisidi]

I don't remember any in universe reason for the wall. It seems like the wall is there for out of universe reasons, to prevent you from seeing the devasted FFXI reference before the story sends you in that direction.

[Cleretic]

Okay, so I've checked the Sightseeing Log, all of Heritage Found's yellow quests, and all the FATEs, and found no direct mention of it; it might be mentioned by errant NPC dialog, but that's a nightmare to check.. In the MSQ the only mention of it I can find is Wuk Lamat calling it 'the big wall'. All of this is a little disappointing, but not too surprising; sometimes something just doesn't get an explanation like that.

But that means the best we've got are context clues, and there is a marked difference between the different sides of the wall; on the east is Yyasulani, the remains of the trainyard, and the Thunderyards, including what appears to be an abandoned mine that Oblivion use as a hideout. On the west is Archeo Alexandria, the more current mining operation of Electrope Strike, and the Crackling Chasm that we're told came from a weapon in the Storm Surge. That's led me to two conclusions, both of which could be simultaneously true.

1. The wall separates the 'civil' part of Heritage Found from the 'wild' part. Almost all of the active, present-day civilization is going on on the eastern side, so the wall might be a simple city wall, separating the safe(ish) area everyone lives and works from the largely untamed countryside; the exception, Electrope Strike, is what you'd expect to be out there, a mining operation. So in that case, the wall is acting similarly to Ul'dah's walls, which are also never explained.

2. The wall is separating Alexandria and Yyasulani. Or rather, at some point was. Notice that the only thing of Tural on the west side are a few ruined buildings, and that there's nothing pre-modern Alexandrian on the east. ...and, if you're especially observant, notice that the geography on each side of the wall doesn't line up; the Turalian side is fairly stable, comfortable ground going right up to the wall, while the western side are sheer cliffs that mostly drop before the wall. There's even some rivers that stop at the wall on the Alexandrian side, with no evidence of ever having flowed on the Turalian side. While some of that could plausibly be the mining operation, my eye says that this is where the dimensional grafting that created Heritage Found got really messy, and the wall might literally be there to denote sort of a 'border' (and maybe a safety thing so nobody falls in). We know that old Alexandria sank; the wall might be denoting the line where it fell.

[Minali]

I don't remember any in universe reason for the wall. It seems like the wall is there for out of universe reasons, to prevent you from seeing the devasted FFXI reference before the story sends you in that direction.

Oh absolutely, I'm pretty sure that's the correct Doylist answer to that question. Would hardly be the first time. ^^ I'm looking more for an in-world reason though.

Okay, so I've checked the Sightseeing Log, all of Heritage Found's yellow quests, and all the FATEs, and found no direct mention of it; it might be mentioned by errant NPC dialog, but that's a nightmare to check.. In the MSQ the only mention of it I can find is Wuk Lamat calling it 'the big wall'. All of this is a little disappointing, but not too surprising; sometimes something just doesn't get an explanation like that.

Wow, thanks for investing all that effort! Didn't even think of checking the FATE texts.

But that means the best we've got are context clues, and there is a marked difference between the different sides of the wall; on the east is Yyasulani, the remains of the trainyard, and the Thunderyards, including what appears to be an abandoned mine that Oblivion use as a hideout. On the west is Archeo Alexandria, the more current mining operation of Electrope Strike, and the Crackling Chasm that we're told came from a weapon in the Storm Surge. That's led me to two conclusions, both of which could be simultaneously true.

1. The wall separates the 'civil' part of Heritage Found from the 'wild' part. Almost all of the active, present-day civilization is going on on the eastern side, so the wall might be a simple city wall, separating the safe(ish) area everyone lives and works from the largely untamed countryside; the exception, Electrope Strike, is what you'd expect to be out there, a mining operation. So in that case, the wall is acting similarly to Ul'dah's walls, which are also never explained.

I suppose to some extent it's a hen and egg situation. No matter if it was originally build for protection, it certainly fulfills the function of a protective wall.

2. The wall is separating Alexandria and Yyasulani. Or rather, at some point was. Notice that the only thing of Tural on the west side are a few ruined buildings, and that there's nothing pre-modern Alexandrian on the east. ...and, if you're especially observant, notice that the geography on each side of the wall doesn't line up; the Turalian side is fairly stable, comfortable ground going right up to the wall, while the western side are sheer cliffs that mostly drop before the wall. There's even some rivers that stop at the wall on the Alexandrian side, with no evidence of ever having flowed on the Turalian side. While some of that could plausibly be the mining operation, my eye says that this is where the dimensional grafting that created Heritage Found got really messy, and the wall might literally be there to denote sort of a 'border' (and maybe a safety thing so nobody falls in). We know that old Alexandria sank; the wall might be denoting the line where it fell.

Oh wow, very good points! Actually that makes me wonder if the wall has been there prior to merging after all, just not at the Source but at the 9th. Electrope was entirely unknown on the Source, so when they're mining it near the Alexandrian ruins now, the mines have to have been from the 9th as well, and when you fly over the Crackling chasm, you can see the raw electrope ore bordering the wall in some places. Unless it replicates itself by whatever means, chemical reactions, ambient aether, it stands to reason it would have been next or close to the wall originally. In the Alexandrian dungeon there is also a huge bridge near the last arena from the same material. And when you look into the distance in that part, you see a very rough landscape.

However, the water is a good point. I failed to remember Alexandria sank prior to being merged and attributed the water in that side of the wall to Yyasulani (and the water inbetween the cracks a result of the damage caused by lightning or the whole merging process, meaning it was once one big plateau with ocean around it and then things got messy, as you said.) The ocean carries saltwater fish from before the dome though, according to the description of the Yyasulani Bowfin, and we know from our time at the First that areas tend to have certain similarities. So the ocean might have existed in both places, as would be the wall's function to block it off, together with the rest from the Alexandrian ruins. Not so much the river on the Turali side though, this looks completely different as you said.
It makes me wonder what the 9th looked at the Yyasulani side of the wall. The building "harvesting" lightning energy is undoubtedly Alexandrian, but the Source has been part of several calamities, a bunch of lightning rods surely would have been stuck in environment or toppled over if they'd be ported with the rest and end up "standing" in the air. The general technology was probably not new but I could imagine they set it up/readjusted it after the fact. And the fields are new, of course, as was pointed out in the story plenty...

I suppose the logical conclusion is that the wall wasn't build after merging but was moved there immediately, likely for separating old Alexandria from whatever else there was, from the damaged dangerous parts but also simply from the rest being flooded. I can work with that. Thanks a lot! ^^

[Cleretic]

Wow, thanks for investing all that effort! Didn't even think of checking the FATE texts.

For what it's worth, to outline my process on this: Gamer Escape has all of that text except for idle NPC chatter, so FATEs and sightseeing log were easy to look at. Yellow quests are a bigger load, but I could scan pretty readily for any relevant words; the wall doesn't have a name, so it'd be referred to as 'the wall' or something, and nobody ever says that (nor do they say a proper name that might be the wall's name). The MSQ was harder and I had to rely on general memory to figure out where to look; none of the 'walk around the Outskirts' quests mentioned it, so I went to the only quest that I knew did refer to it: the one where we're following Gulool Ja. Which is the time Wuk Lamat acknowledged it, but that's all.