Part of the "Megastructures: The Visual Encyclopedia" book project, on sale now at http://www.neilblevins.com/megastructures/megastructures.htm
A Shellworld would be an artificial planet made up of nested spheres, like a Russian Matryoshka doll. Each sphere would be called a shell, or level, like a floor in an enormous skyscraper. Shellworlds could have a single layer, or dozens of layers.
The shells would be held up by enormous towers, which could also serve as elevators for movement between levels, and could pump out waste heat to the upper layer.
Each shell could be set up to have a completely different environment and atmosphere in order to accommodate different lifeforms on the different levels.