As the Earth died around them from ecological exploitation and violent wars, one city kept itself armored behind a force field dome. While this protected it, they would eventually run out of non-renewable supplies, leading them to a daring experiment to plane-shift themselves into another reality whose world was younger and still ecologically viable.
They accidentally shifted their entire city into a time and world where dinosaurs had not yet gone extinct. Colonizing this new and dangerous world took generations, and varying levels of technology now propagate across the surface of the world. Some live fairly simply, as farmers or townsfolk that have the dinosaurs domesticated as genetically-modified beasts of burden.
Other factions are at regular war with each other for choice pieces of territory. They too command modified dinosaurs, but use T. rex battle mounts plated in armor and equipped with automatic cannons. These factions are lead by tyrants and warlords, hoping to dominate ever-larger kingdoms.
Trade through the dangerous dinosaur-filled wilderness is facilitated by airships. Though pterosaurs are dangerous, they avoid aircraft of such size (especially if they are defended by sharpshooters), and so they serve as public and private transport of people as well as cargo freight.
(I do not own the images in the above collage, and all copyrights belong to their respective owners.)
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