It basically boils down to a question of topology. There is no environment on Earth that is more full of durable, irregular nooks, crannies and crenellations than the human built environment. Any irregular terrain is difficult to assault, if defended resolutely. The more irregular the more difficult.
Maybe some natural geographic area with massive cave systems or something, but even then I doubt it. Not when you take into account all the tunnels, subtunnels, sub-subtunnels, and basements and outbuildings and so on of a modern urban environment.
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