Kelim, Chapter One, Mishnah Seven
Cities that are walled are holier, for metzoras must be sent out of them and a corpse, though it may be carried about within them as long as it is desired, may not be brought back once it has been taken out.
There are two halakhot that make walled cities holier than non-walled cities.
First of all, metzoras must be sent out of walled cities. This is based loosely on Leviticus 13:46 which states that a metzora must be sent out of the “camp.”
The second halakhah is that a corpse may not be brought into a walled city. However, a corpse that is already in a walled city need not be removed quickly.