The Floating Military City State of Atlantis

When Plato invented his fictional warfaring city-state of Atlantis in his work, "The Republic", in 375 B.C. as the evil villain for his noble city-state of Athens to defeat, he could not have imagined that over a thousand years later diabolical forces would attempt to build an exacting replica of the mythological city and pit its armies and navies in mortal combat against the very real city of Athens just to find out what really would have happened in the fictional battle.
If a few modern scholars are to be believed, it is entirely possible that the above nightmare scenario is what is being described in the 11th century tales of Ingrid Malmann, also known in legend as, Valkyrie, Chooser of the Slain.
In the Tales of Valkyrie, after taking control of her husband's fighting forces and improving their sea vessels, (and of course after defeating sea serpents and other minions of evil using her small navy), they encounter a floating armada / island in the Atlantic Ocean. Upon landing and greeting the natives, they find the island to be ruled by a confederation of kings who largely only rubber-stamp commands given directly by none other than the ancient god of the Sea, Poseidon, who lived on the island very much in the flesh, in the form of a very large yet well proportioned giant of an athletic late-middle-aged man.
Poseidon and the locals made no secret of their plans to attack Athens, aside from ensuring that their floating island armada was significantly faster than the vessels of the heroine, and leaving them adrift in the ocean, unable to send any warning to Athens that would arrive in time to make any difference.
Valkyrie argued with Poseidon that the battle between the two of them had been decided roughly 4000 years earlier in a contest so famous that the story was literally engraved in marble in the largest temple to Athena in the city of Athens. In fact, the naming of the city bespoke who was the winner of that legendary contest.
This Poseidon, who Valkyrie considered to be an imposter, would hear nothing of her arguments, and insisted that now times had changed, that he now led a navy of tritons the likes of which dwarfed his previous grandeur as simply the king of the oceans.
Similar to 20th century James Bond pulp novels, Poseidon allowed Valkyrie and her men significant freedom to explore the island (sometimes unattended) and lavished fine foods, drinks, and entertainment upon them. Rather than treating them as potentially hostile military forces and imprisoning them, he appeared to be trying to woo them to join his forces. It could be considered that the god of all seas held such inherent vanity to assume that any sailors of any vessels worshipped him without exception, and that any loyalty to Athens they might have previously held would disappear when pitted against the possibility of angering the god of the sea by disobedience of a direct order.
Regardless of why Poseidon made the strategic blunder of allowing them access to observe the armada, Valkyrie soon concluded that the entire sailing settlement, the floating fortress with all its components, and every living inhabitant had been biologically grown over the course of the previous one year. She claimed to her husband (who often served as the unenlightened plot explication character in her stories) and his troops, so her tale goes, that microscopic machines from a much more advanced society had collected and disassembled floating debris, krill, and other oceanic lifeforms on a large scale and reconstructed their structure in the form of the buildings, foundations, propellers, and even the tritons and mermaids which they now beheld. The settlement included living quarters, mess quarters where fish and fine fruits and vegetables were served, and sparring grounds. Valkyrie claimed that even this hulking (but otherwise very human-looking) Poseidon was constructed from parts of lesser fish and crustaceans.
The legend of Valkyrie, set in A.D. 1000, written for the first time around A.D. 1100, may be in fact the first true hard science-fiction story ever recorded. With today's 26th century technology, such a floating monstrosity certainly could be built, but the thought of actually doing so (and causing catastrophic loss of life in the ensuing battles) is today thankfully very firmly stuck in the realm of fiction.

Type

Large city