Mishna, Baba Batra 1:5

A resident of a courtyard may be compelled [by the rest] to [contribute to] the building of a porter's lodge and a door for the courtyard. Rabban Shimon b. Gamliel, however, says that not all courtyards require a porter's lodge. A resident of a city may be compelled to contribute to the building of a wall, folding doors and a cross bar. Rabban Shimon b. Gamliel says that not all towns require a wall. How long must one reside in a town to be counted as one of the townspeople? Twelve months. If, however, one buys a house there, one is at once reckoned as one of the townspeople. [Soncino translation]