Denizens of Atlantis

Plato's original writing of the citizens of Atlantis only went so far as to describe a confederation of kings, each responsible for a portion of land and its population. In Plato's story, this form of government was not the most noble, so when it waged war on the more noble republic of Athens, the Athenians won, and apparently the gods (uphappy with how the government of Atlantis turned out) washed the country's land with water, turning it into quicksand, sinking all the inhabitants, and leaving the muddy location unusable by farmers and sailors alike.
But when amoral meddling scientists from a dramatically more technologically advanced realm arrived around A.D. 950, they postulated that perhaps if the story hadn't been controlled by Plato (and indirectly by his predecessor, Socrates, the inventor of Logic) that the actual battle might have turned out very differently.
These scientists, (who were referred to as The Norns by their adversaries, The Fates, who found this interference in the natural progression of a less advanced society to be morally repugnant), used their microscopic machines, Nanobots, to fabricate a reproduction of the fabled Atlantis close to A.D. 1000. Of course, there was no land mass and no vast expanse of quicksand as mentioned in Plato's allegory, so they had to take some creative liberties.
Out in the Atlantic ocean, their machines harvested all debris and swimming life forms as raw materials for the construction. Not only did their machines build the floating foundation and appropriate housing for the fabled confederation of kingdoms, but they had the biological knowledge to fabricate its kings and citizens as well.
It was an obvious choice to build architecture similar to that of Athens, which allowed troop training maneuvers to be practiced in a controlled setting prior to the invasion of Athens. But, growing soldiers was not so simple.
Tritons and Mermaids were manufactured cell-by-cell on a mass scale on this floating island, with its powerful means of locomotion hidden deep underneath. But no matter how advanced the Norns' technology, building sentient life forms in the fully grown adult state with memories of language, customs, and social norms was beyond even them. These full grown people with the minds of babies needed a strong leader and teacher.
The Norns drew on much earlier legends of the contest for Athens, in which Athena and Poseidon competed for control of the valuable city state. It seemed a perfect fit that Poseidon should be the general / admiral in charge of the Atlantean forces. Constructing their Doppleganger in Chief was a chore requiring human brain matter and personal training by the Norns, but after several months, their manufactured Poseidon was conversational enough and trained in military tactics and teaching techniques.
Then Poseidon trained one legion of Tritons and Mermaids in simple one-word commands and a sign language capable of expanding as needed to cover all of the concepts of a society built entirely of full-grown adults preparing for war. The initial legion of Tritons and Mermaids became the kings and queens of the following personnel as they were manufactured, training their troops as Poseidon had trained them.
Eating and drinking was automated as designed by the Norns, with seafood cleaned and cooked by nanobots and sometimes restructured and flavored into reasonable imposters of poultry, pork, and beef.
All day long every day, for the citizens, combat training was their work, their sports, and their entertainment. Only the Kings and Queens had to learn personnel management and the various "carrot and stick" methods of manipulating and controlling a population.
Natively born and raised people see a sadness to the manufactured ethnicity built by scientists for one solitary goal with little regard for the value of the individual, but the poor creatures knew no other way of living. At the very least, they could say that they were fully occupied and well fed.