<1. While it does eat planets, it takes a long time and breaks it off piece by piece.
2. It has mechanics in place to convert planetary materials into fuel and other substances. Emphasis on other substances.
3. It has lasers and everyone knows the best mining is done with lasers.
So why does it have a city on top? Well you need homes for the crew. Clearly this was all built before Calus was exiled as all the architecture isn't rugged on the outside.>
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Calus is probably an engineer of some sorts, or at least took a ton of engineers with him when he was exiled. He clearly made or planned the development of the Leviathan before being exiled and then made his Gary army in a part of the Leviathan at some point and even used one to fight Val Ca-our. Makes you wonder if him stealing all of the smart Cabal is the reason why the current Cabal are dumber than rocks.