פירוש על בבא בתרא 1:5

כופין אותו – [we force] the member resident of the courtyard who doesn’t want to assist the members of the courtyard.

They compel [a partner in a courtyard to contribute to] the building of a gate-house and a door for the courtyard. Rabban Shimon ben Gamaliel says: “Not all courtyards are fit for a gate-house.”
They compel [a resident of the town to contribute to] the building of a wall for the town and double doors and a bolt. Rabban Shimon ben Gamaliel says: “Not every town is fit for a wall.”
How long must a man dwell in a town to count as one of the men of the town? Twelve months. If he has purchased a dwelling place he immediately counts as one of the men of the town.
Mishnah five deals with the obligation of a person who shares a courtyard with another to help in building certain parts of the courtyard. The second half of the mishnah deals with the obligations that residents of a town have to participate in the costs of building the public structures in the town.
Section one teaches that a person who holds joint possession of a courtyard can be compelled to share in the costs of building a gate-house (where a guard would sit and protect the homes attached to the courtyard) and a door. Since these are necessities of a courtyard, one must participate in their cost. Rabban Shimon ben Gamaliel points out that not all courtyards need a gate-house. Assumedly smaller courtyards can be protected without building a gate-house. Therefore, a person would be obligated to help pay for building a gate-house only as long as the courtyard was fit for one.
Section two discusses similar laws with regards to people who live in a town. Just as in modern society people pay taxes in order to pay for the building and upkeep of town property, so too in the times of the Mishnah people had to jointly pay for the building of a wall, double-doors and a bolt to help protect the town. Again Rabban Shimon ben Gamaliel points out that not all towns are fit for walls. If the town was not a one which would usually have a wall, the residents are not obligated to share in its costs.
Finally, section three discusses how long a person must dwell in the town in order to be an official resident and thereby be obligated to pay for building its wall. There are two criteria. First of all, if he dwells in the town for twelve months he is obligated to pay. Second, if he purchases property in the town, he is immediately obligated, even if he doesn’t dwell there.

לבנות – [to build] for the courtyard a gate house to be a guard for the opening who sits there in the shade and to distance the members of the public domain from looking into the courtyard.

ודלת – for the gate of the courtyard.

לא כל החצרות ראויות לבית שער – a courtyard which is not close by to the public domain is not worthy of havng a gate house. But the Halakha is not according to Rabban Shimon ben Gamaliel for even if it is not close to the public domain, there are times when the masses pressure and enter [and come in].

לא כל העיירות ראויות לחומה – a city which is not close by to the border of enemies does not require a wall. But the Halakha is not in accordance with Rabban Shimon ben Gamaliel, for even if it is not close by the border of enemies, it needs a wall, for sometimes, it happens that invaders come.

ויהא כאנשי העיר – to bear with them the burden/yoke.

שנים עשר חדש – but now that they are unsteady, the general practice is thirty days.