(Mishnah): (Any resident of the courtyard must share the costs of the courtyard. Therefore,) we force him to (share the costs to) build a gatehouse and a door to the courtyard.

Rabban Shimon ben Gamliel: Not every courtyard is worthy of a gatehouse.

We force him to build a wall, doors and a lock for the city.

Rabban Shimon ben Gamliel: Not all cities are worthy of a wall.

Question: How long must a person live in the city to require him to share the costs?

Answer: 12 months.

If he bought an apartment there, he is immediately considered an inhabitant of the city.

(Gemara) - Rebbi: (Commenting on the Mishnah's law that residents of the city must pay for the wall, the doors and the lock) That is not the way to obligate people; rather, whoever makes his livelihood from the city must pay, as the pasuk states (Mishlei 18, 11), "A rich person's treasure is his city of strength etc.''