Floating City

The Floating City (name to be determined) was built during the First Age by a trio of powerful wizards. During the First Age it was held aloft with magical enchantments powered by a series of enormous magical batteries. The batteries also powered a magical concealment engine which was able to cloak the city to keep it hidden from prying eyes.
When the God War started, the rulers of the city (still the three founders), moved it into a pocket realm to further protect it from detection. There was a thin cord of energy that kept it tethered to the mortal realm and kept the batteries charged from the ambient magical energy of the realm.
When Dáblo absorbed all ambient magical energy into herself, it caused a feedback pulse into the city, temporarily super-charging the batteries but also pulling the city back into the mortal realm. The feedback pulse killed thousands in the city, including one of the founders, as magical energy arced like lightning across the contuctive surfaces of the city. The batteries remained charged, but could no longer recharge. The city was able to stay aloft and hidden through most of the chaos that ensued. As the batteries discharged, however, the remaining two founders located a safe place to hide the city (where?) and landed it safely.
The city remained hidden with a minimal amount of magical energy used from the still partially charged batteries to keep it hidden and to keep the critical magical functions of the city operational. It stayed under cover through a large part of the Second Age.
The resourceful inhabitants of the city kept watch over the development of civilizations of the centuries. When fusion power was developed, they quickly adapted it as a replacement to for the now drained magical batteries. They found that with the right configuration of nuclear and electrical technologies, many of the previous functions of the city could be brought back online. This included the cloaking and floating abilities of the city. Thus, the city was safely in the sky and hidden when The Event occurred.
The city's power cores have continued to remain online to the present day in the Fourth Age. Thanks to the efficiency of the fusion generators and the ready availability of fuel (deuterium and tritium from sea water), the floating city will likely remain aloft for millenia, unless sabotaged or otherwise attacked.
The return of magic to the world has allowed some of the magical systems that had been offline for millenia to come back online. However, the great magical batteries are unlikely to ever be fully recharged due to the reduced levels of ambient magical energy as compared to the First Age.
The residents of the Floating City continue to keep to themselves. The city is largely self-sufficient and what supplies they need they tend to acquire surrepticiously. They do maintain an active spy network on the ground so they can be forwarned against possible attacks against them. But only a handful of the inhabitants of the world know of their existence. The gods are aware of their existence, but choose to ignore them for now, as long as they remain primarily isolated.

Type

Acropolis / Citadel