It's a topic I've thought about in the context of very large structures (with Germany-sized living areas)

Insofar as the structure has to hold it's own "weight" it can't be very thick and I'd be worried about artillery making a hole in it. A game changing concept I haven't seen in the literature is that if you had good enough bearing the rotating structure wouldn't have to hold its own weight and could rotate more quickly (be larger) and also be thicker. It is still hard to believe such a structure is going to have a lot of up and down relief or anything within an order of magnitude of the 0.2 cubic kilometers of ocean per person that we have on Earth.