in the recent acoup post about the fate of cities after the fall of western rome he mentions that Arles, which was once a real city, declined so much that the city moved inside its former amphitheater and used it as its city walls!

i dont think ive ever seen this in modern post apocalyptic media, which is too bad cuz its so appropriate! our society is full of absolutely massive stadiums that could easily protect a small village. a lot of the older ones are concrete so theyll hold up, but the ones made of metal or with those huge closeable roofs will probably collapse. maybe good for salvage, and then you can inhabit it when they remove the metal..