The cities assigned to each Levitical clan were geographically grouped. Kohath in the South, Gershon in the North, and Merari in the East and West. Aaron was of the Kohath clan, so his descendants received cities in the South, closest to Jerusalem, although the city was still occupied by the Jebusites and the Ark wouldn't be moved there for more than 300 years. The Levites had the same stores of wealth that the other tribes had, but their wealth had to be concentrated around their 48 cities.
Joshua 21:1-42