Bava Basra, Chapter 1, Mishnah 5. The Mishnah continues to discuss the additional things a person builds around his courtyard together with the rest of the members of the courtyard, things that provide privacy and even security.
A Gatehouse and a Door for a Courtyard:
The Mishnah says: "Kofin oso livnos beis shaar vedeles lechatzer" - the members of the courtyard may compel any one of them to share in the cost. A gatehouse is a structure that encloses the entrance to the courtyard and prevents those standing outside, in the alleyway or the public domain, from peering inside and seeing what is happening in the courtyard; a sort of additional facade. Likewise, a door is built that can be locked, preventing people from entering at night. All members of the courtyard are obligated to pay for both of these, and whoever tries to evade this - we compel him.
"Rabban Shimon ben Gamliel omer: lo chol hachatzeiros re'uyos leveis shaar" - in his opinion, not every courtyard needs a gatehouse. Many courtyards are recessed deep inside, at the end of the alleyway, and passersby in the public domain cannot see into them: the distance is too great and the angle does not allow it. And even though someone who approaches the end of the alleyway could peek inside, who bothers to walk to the end of the alleyway just to look into someone's courtyard? Therefore, in courtyards that are far from the public domain, the members of the courtyard cannot obligate their residents to pay for a gatehouse.
However, the halachah follows the Tanna Kamma: in every courtyard, all residents can be compelled to pay for both the gatehouse and the doors.
A Wall, Doors, and a Bolt for the City:
The Mishnah further states: "Kofin oso livnos la'ir chomah vedelasayim uveriach" - all residents of the city can be compelled to participate in building them:
"Chomah" - a wall surrounding the city.
"Vedelasayim" - gates that protect the city.
"Uveriach" - a bar that locks the gate from the inside, so that they cannot break in.
All of these are meant to provide protection for the city's residents, and everyone is obligated to pay for them.
"Rabban Shimon ben Gamliel omer: lo chol ha'ayaros re'uyos lechomah" - in his opinion, towns situated on the border indeed need a wall, but towns in the middle of the country - who would come to break into them? They do not need walls, and therefore their residents cannot be compelled to pay.
The halachah follows the Tanna Kamma: every city needs walls, since there are problematic people and thieves all over the country, and therefore every city could reasonably need a wall, and anyone who is a resident there is obligated to pay.
Who Is Considered a Resident of the City?
It turns out that the question is who is considered a resident. The Mishnah asks: "Kammah yehei ba'ir vihei ke'anshei ha'ir?" - how long must a person live in a city to be considered one of its people? The Mishnah answers: Twelve months. However, someone who purchases a house to live in the city becomes a resident immediately, as he demonstrates his intention to stay there. Therefore, he must pay his proportional share for the wall and similar expenses immediately upon purchasing the house, even if he has only been there for a few days and not twelve months.
The Mishnah therefore focuses on people who stay in the city only for the summer or who pass through temporarily, and they are not obligated to pay for the walls. As for someone who rents a place for twelve months - there is a dispute about this, and this dispute continues all the way to the Shulchan Aruch, whether one who rents for twelve months is considered a resident obligated to pay for the walls.
It is worth noting the words of the Gemara: Although everyone is obligated to pay for the walls, Torah scholars - who are obligated in certain types of taxes, such as paying for road maintenance, because they benefit from them - are exempt from paying for the walls. This law has far-reaching implications, and for our purposes, it is enough to note the halachah: The Torah scholars in the city are exempt from paying for the wall.
In summary: We learned in the Mishnah that the residents of a courtyard can be compelled to build a gatehouse and a door, and the residents of a city can be compelled to build a wall, doors, and a bolt. The halachah follows the Tanna Kamma in both cases, unlike Rabban Shimon ben Gamliel who differentiates between different types of courtyards and cities. We also covered the definition of a resident who is obligated to pay - one who lives there for twelve months, and one who purchases a house immediately - as well as the exemption of Torah scholars from paying for the wall.