A concept for another of my personal projects (yes, I have plenty of those :) called 'Absolutum'. The story, a pretty complex one, takes place in a futuristic fantasy-like kingdom in totalitarian setting. The world, full of bio-engineering, meta-alchemy and psionics is ruled by the Empress, a quasi-immortal with powerful psychic powers thanks to her longevity and numerous mutations. The magical substance called Arcalum is the base of her power. Arcalum is used to run machinery and weaponry developed by royal meta-alchemists as well as a high-energy fuel source used in air-fregates. The rule of the Empress is eternal and unthreatened. Or so it seems.
The city, with the main 'stem' divided into dozens of levels, is built among the remnants of giant nests of ancient dragon-like creatures, extinguished many eons ago. The lining of their partially preserved cocoons was used by the first meta-alchemists to create original supply of Arcalum.