All-World is divided into three regions; In-World, Mid-World, and End-World. The geography is widely varied. It includes deserts, mountains, rolling plains and vast wastelands. All-World is said to have "moved on." This seems to mean that where there was once great order there is now little if any. For example: the sun does not always rise in the correct place, and at times it will not even cross the sky in an entire day; the compass directions themselves are at drift, what is East today, could be Southeast next week, and next year may be full-on South. According to Blaine the Mono, a talking monorail that functioned throughout Mid-World, his "Slo-Trans" engines were supposed to last for millions of years, but were already faltering after a few thousand. However, since time appears to run at different rates all throughout All-World, and given Blaine's dementia, this assessment may not be dependable.
It seems that an extremely advanced civilization once existed in All-World, a parallel United States, known as the Imperium. This can be inferred from the high degree of similarity between Old Ones (the name for these ancient people) architecture, automobiles, clothing and the fact that an Old Ones military outpost used an everyday phone modem and Microsoft products. They might have spoken English.
The civilization of the Old Ones is inferred to have collapsed because of the replacement of magic, which could last forever, with technology, which would disintegrate if left unattended, but which nevertheless was allowed to be responsible for maintaining the Beams of the Dark Tower.
The Beams are six invisible forces connecting the edge of the world/universe to the center. These Beams are the primary sources of force in All-World and they maintain order. Failure of the Beams causes changes in physical and astronomical constants, which causes chaos in nature, as well as in civilization. There were six Beams with twelve Guardians, one for each "Portal" (the end of a Beam) arranged like the spokes of a wheel with The Tower at the center. (see also: The Waste Lands). Guardians were based on novels like Shardik (for the bear) and Maturin for the turtle (Patrick O'Brian's character Dr. Stephen Maturin was a naturalist who named at least one species of turtle).
The Old One civilization used technology to maintain the Beams of the Dark Tower instead of magic, and sought to literally control reality. The final blow for the Old One civilization seems to have been nuclear war, biological warfare, chemical warfare, and something "else" that is hinted to be more horrible/powerful than those three, but it is suggested in The Dark Tower: The Gunslinger Born that it was the final attempt to destroy the Dark Tower itself and rebuild something in their own image as the nexus of space and time. It is not known when such wars took place (but long ago enough that it is in the ancient past, and that the gene lines are finally starting to breed true, or "threaded"), why they took place, or even between which nations or organizations such wars were fought. It is widely hinted the Crimson King was ultimately working behind the scenes, manipulating people and events to bring about civilization's destruction, since such destruction serves his ultimate ends.
The All-World of most of the Dark Tower series is sparsely populated and dangerous, filled with mutants both human and animal, and vast swaths of land are irradiated. Demons and robots are to be found, as well as Taheen, who are in the employ of the Crimson King. Governments do not extend beyond the town/local level. Resources are scarce, and relics of the Old Ones' technology exist mainly in perverted, deranged, or damaged forms that threaten instead of helping the last remnants of civilization.