Outside
The Bonefields
Living bone is, weight for weight, stronger than concrete.
An enterprising bio-engineering company attempted to use it as a construction material, growing fields of straight bone 'trees', supplied by vast mechanical pumps filled with artificial blood. The pumps could be powered by solar panels installed on building roofs, and creating the artificial blood was a good way to use up waste food. A prototype industrial neighbourhood was built, with buildings with bony skeletons, strong and flexible.
After the apocalypse, with no-one to maintain them, the company labs and the adjoining experimental neighbourhood grew out of control, sustained by the dead biomass that accumulated in the area. The buildings split apart into insane masses of spiked bones growing in all directions. Eventually the buildings 'died' and the bones desiccated into shards, making an area that's too dangerous to traverse because you're bound to fall through something unstable and get impaled or buried in bone dust.