A bit of a spoiler since it includes an endgame area so beware, or not since it's just a speculation and an area that you've found early on.

So I've just arrived at the endgame of my playthrough and found myself on the seafloor shrine now fully revealed, and as a jest I thought "wow, imagine if this was Gran soren lol" and that small joke made me double check, the building positions, the hole in the middle, the FUCKING MAP, it was Gran soren after all 😂

Then I thought of "what if there are other areas in the first game here in this game?" So I checked the map again, first went to Sacred arbor since the water there is gone as well, only to find out there's no resemblance of the old game there, thought there would be because of the ruins, but then again Elves are a very new thing.

So I checked the map again and remembered the shadow fort, heck I even followed the route you'd usually make on foot in the first game and found myself on the Ancient Battleground, it was too perfect that it hit me "holy shit it's literally shadow fort... But smaller?" It was weird how the old game's area are smaller but it wasn't too bad, the layout in the map itself of the fort is nearly the same, there's even a cave at the very back that leads you to somewhere much like the first game did.

So I'm pretty sure that the Ancient Battleground is the Shadow fort, makes you think that if there are other areas of the old game :))

Ps. I got lazy to rewriting this, but I also remembered that bluemoon tower is also in the game, same exploration as well to the first game