The Lands Between is in complete disrepair by the time we find it in Elden Ring so you probably never thought twice about the big stone ruins you find randomly dotted about--some cathedral or old castle has fallen apart, right? Easy enough. Mystery solved. Not quite. These ruins are from Crumbling Farum Azula, the giant floating temple at the end of the game.
Crumbling Farum Azula was initially a royal city ruled by Dragonlord Placidusax, hinting at a dragon civilisation that predates the era of the Erdtree (not unlike how dragons ruled the world of Dark Souls before Gwyn and the other lords found the first flame). Some theorise that this city was located somewhere in Limgrave, either in the water, on the land, or in the sky (meaning it has always been floating), but the endless tornado moved it. Interestingly, the Bestial Sanctum in Caelid shares similar architecture, so there may even be a connection between these locations.
One theory posited by Woodie626 is that the city was not actually from the Limgrave region, but that "The whole thing was torn from the Bestial Sanctuary before it became the floating city". This would explain why giant ruins are scattered across The Lands Between, why the architecture is similar, and why the bridge nearby is called the Farum Greatbridge--it even has a dragon defending it. One of the earliest bosses is also called the Beast of Farum, which along with statues and enemies found in Crumbling Farum Azula, all but confirms a coexistence between them and the dragons of this primordial city.
The ruins also have fragments scattered across them that you can pick up. If you read the item description, it says that they are "found near places where ruins have fallen from the sky [...] These shards of stone are believed to have once been part of a temple in the sky". That describes Crumbling Farum Azula to a T. The big mystery then is where the city came from, if it was always floating, and what its connection to the Bestial Sanctuary is.
Of course, inside the city, we can find statues depicting beasts and dragons, with one even theorised to depict the Gloam-Eyed Queen. As per the Godskin Swaddling Cloth, she "cradles newborn apostles" who will soon "grow to become the death of the gods". We do find a Godskin Duo in Crumbling Farum Azula, as well.
It's an interesting location steeped in lore and mystery, being a floating city once ruled by dragons, and fans are hoping that we'll find out more in the DLC. But for now, there's plenty to unpack in the base game.