The "Ristaccia Ruins", or "Ruins of the Skyship", as shown on the Aria Te'Laria world map.
The Ouroboros Ark was a flying vessel (空船, sorafune, lit. "skyship") carrying a database that contained the remainders of humanity as it once lived in the AD World, serving as the culmination of the eponymous Project Ouroboros.
Following the successful launch of the Ark, and after an undetermined amount of time, Joah, one of the scientists working on Project Ouroboros at the time of its completion, managed to detach from the cluster, seemingly dissatisfied with the essential condition of its existence, which was the total suppression of meme — and in effect, of all mental and cultural processes that defined humanity. As such, he set out to "reset" the Ark by creating L'erisia, the embodiment of meme.
This event led to the recreation and fragmentation of the AD World humans into the Eight Ancient Races. From an outsider's frame of reference, the Ark "fell" on the continent of Galukia, after which the Ancient Races uncovered Ristaccia from its remains, in turn kickstarting the events of the Epic of Zektbach.
The site hosting these remains is alternatively called the Ristaccia Ruins (on the world map), or the Ruins of the Skyship (most notably by L'erisia herself). Joah's memos, the records of his activity after dissociation from the Ark and up to the moment of its fall, are still present on the site. As the information they contain constitutes a threat to L'erisia's existence, she seemingly prevents anyone from reaching them - as was the case with Lien, who briefly discovered them only to die at the hands of Shamshir.
L'erisia also appears to personally reside within the ruins : following Malchut's contact with meme and her distress in the face of Kukaru's death, she invites her there, and a battle occurs between them, as depicted in Raison d'être. Brief descriptions of Chapter 8 also include A'Milia returning to the ruins to face L'erisia again.