Cites are large permanent human settlements populated by many buildings and people. Over half of all people on Earth live in or near cities. Because of the viral nature of the zombie condition, the dense population of cities makes them incredibly dangerous. Cities are usually the first areas the infestation is prominent, and the last to be reclaimed, should a movement to retake territory be undertaken. Cities are where the most zombies are, but they are also where the majority of civilization's supplies are kept (though without society's commercial infrastructure to produce them, this becomes a finite number in a zombie apocalypse).
Looting will take place immediately since law enforcement and the military will have been overrun. Panic may lead to banditry, massive traffic jams, and the general breakdown of law and order. As such the earlier one can leave a city, the safer one will generally be. However, with thousands (perhaps millions, depending on the city) of people sharing the same notion, cities can quickly become death traps with no viable escape until the population thins.
If the city is being used to gather refugees this may become a problem as this is a likely scenario to play out. considering all this, retaking the city would become a great hazard only achieved through strict combat doctrine and enough ammunition. A prepared city can be fortified with walls, fences and other terrain features like rivers or mountains.
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