Though not much is known about the world in which The Last Guardian is set, much can be discerned from what remains of the crumbling ruins and from its respective geography.
A place shrouded in mystery—and for good reason. The Nest is a massive, crater-like valley surrounded by sheer, impassable mountain walls that can only be bypassed by flight. Where the Nest truly gains its formidable and lonely atmosphere, however, is within its bowels; crumbling ruins, towers, courtyards, bridges, passageways, and temples lay in relatively uninhabited decay.
On the outside, structures straddle deep caverns and any remaining pathways are often treacherous. On the inside, endless tunnels—some of them flooded—offer more puzzling situations.
The buildings and stonework are heavily overgrown by lush, green ivy, sections of forest, and other lesser plant life, such as moss and lichen. It is unknown who constructed these ruins, as well as who may have lived here before or what may have happened to cause their downfall. It is possible that the Master of the Valley was responsible for the Nest’s current dilapidation, although it is uncertain.
The carvings and runes coating these ancient structures can offer insight into the Nest’s history; their overall shape, style, and design highly resemble, if not match, those of the Master of The Valley. One theory is that these runes were implemented by the Master. If so, it might have been responsible for the ruins’ construction. Another theory is that the people who built the ruins happened to share a common language with the Master. In this case, the Master’s involvement is indiscernible.
As the ruins are uninhabited by any known civilization, animals, and other beings have come here to take up residence. These creatures include Tricos—huge, winged, griffin-like beasts—as well as an army of possessed suits of armor called Armored Knights. The most notable being of the Nest, however, is the Master of The Valley, as it appears to assert dominance over the others.
The only one of these inhabitants shown to be capable of leaving the Nest, save for wild birds, are the Trico-beasts. This is probably a significant reason why interaction between the human inhabitants of the outside world and the Armored Knights of the Nest have never occurred within living memory. Similarly, this is a likely reason why the Master's dominion has been limited to the Nest and why the humans of the outside world have not yet fallen under its dominion.
Other, lesser inhabitants include unknown species of birds, salamanders, and butterflies.
The outside world
Little is seen of the world outside the Nest. From brief glimpses, though, it seems to be heavily wooded as far as the eye can see, possessing a temperate climate. Rain and lightning storms appear to be just as common as they are in the real world.
Dotting the forest are man-made clearings containing human villages. The primary layout of these villages seems to consist of a wide-open clearing with houses and other structures hugging the treeline. The technology of these human inhabitants appears basic but perfectly adequate for their lifestyle: homes are lit with oil lamps while outside spaces are lit with torches; weapons include throwing-spears, bows, arrows, and swords; armor may be sophisticated, but from what has been revealed of village defense—men waking bleary-eyed to fend off a beast—it is uncertain.
Little is seen of the local wildlife, but it may be fairly diverse, given the vibrantly green environment.