Gemara

42a (Mishnah): Locking the fence, fencing or breaching any amount in a wall is a Chazakah to acquire a gift or Hefker or to divide an inheritance.

53b (Rav Nachman): If Reuven built mansions on Hefker property (a convert owned it, and he died without heirs), and Shimon put doors on them, Shimon acquires them.

Without doors, a mansion is a mere arrangement of bricks.

57a (Mishnah): Raising chickens in a Chatzer is not a Chazakah. If one sets up a 10 Tefachim wall to enclose his animal, or for his oven or millstone, or brings chickens into a house, this is a Chazakah.

(Ula): Only a Chazakah that acquires Hefker property is a Chazakah in another's property.

Objection (Rav Sheshes): Eating Peros does not acquire Hefker, but it is a Chazakah in another's property!

Gitin 77b: A man was about to die on Shabbos. Rava suggested that he give to his wife (on Shabbos), for a gift, the place where the Get is resting. She should close or open the door there to acquire it through Chazakah.

Rishonim

Rambam (Hilchos Mechirah 1:10): Locking is a Chazakah when the door to a house or Chatzer was open, and the buyer locked it and opened it. He did a usage that helps.

Rambam (Hilchos Zechiyah 2:9): If Reuven built mansions on Hefker property, and Shimon put doors on them, Shimon acquires them. Reuven did not do anything to the land itself. He is like one who made a rockpile, who does not acquire. This fence does not help, for it is too wide and open at the ends. The form of the building is useful only once doors are set up.

Rosh (3:59): A Mishnah teaches that locking is a Chazakah. The Rashbam explains that this is putting a lock on the door. Closing the door and locking it with a key is not a Chazakah to acquire Hefker, for it is merely Mavri'ach Ari (preventing damage). This is wrong. Rava said that a woman can close and open the door to acquire through Chazakah. He did not tell her to affix a lock, for it was Shabbos. Chachamim permitted to acquire from a Shechiv Mera on Shabbos, lest he become frenzied. They did not allow fixing a lock, which is an Isur Torah, even through a Nochri. This is unlike Mavri'ach Ari, for one is obligated to save another's property. To lock a door in front of everyone and prevent them from entering a house shows that it is his. He acquires just through locking. A Mishnah teaches that bringing chickens into a house is a Chazakah. Ula taught that only a Chazakah that acquires Hefker is a Chazakah to acquire from others. We challenged Ula, but not from the Mishnah. We must say that he brought chickens in and locked the door in front of them, and this acquires! However, perhaps this is considered eating Peros, and the Gemara challenged Ula from this.

Rosh (ibid): Later, we say that if Reuven built mansions on Hefker property, and Shimon put doors on them, Shimon acquires them. This connotes that locking is not enough by itself. Do not say that one acquires through erecting doors without locking them. We said that Reuven did not acquire because without doors, the mansions are mere arrangements of bricks, i.e. for one can enter easily just like before the mansion was built. The same applies if he erected doors but did not lock them! Really, locking suffices by itself. The Mishnah discusses erecting doors, for typically the one who erects the door locks it. The Rashba connotes that affixing a lock acquires even without locking. If so, also erecting doors acquires even without locking.

Rashbam (53a DH v'Hashta): 'No'al any amount' can mean that initially one could enter easily, and now one can enter with difficulty. Alternatively, he affixed a lock, which is building. Closing the door and locking with a key does not acquire. It is just a Mitzvah to guard another's property and Mavri'ach Ari. Handing over the key acquires for rental, but this does not apply to Hefker.

Rashbam (53b DH Kama): Without doors, one can enter easily just like before the mansion was built. The completion is primary, not the beginning. A Mishnah teaches that locking the fence or fencing, i.e. erecting doors, is a Chazakah. (It says that breaching any amount in a wall is a Chazakah, but) it does not say that building any amount is a Chazakah.

Poskim

Shulchan Aruch (CM 192:3): How is locking a Chazakah? If one sold or gave a house or Chatzer, and the door was open, and the buyer locked it and opened it, he acquired. Some say that he must lock it with a key.

Beis Yosef (DH v'Da): Those who argue with the Rashbam and say that closing suffices, they require closing with a key. The Rambam connotes like this, and the Rosh in Gitin says so. The Rashbam connotes that locking does not acquire for a renter, buyer or from Hefker. Why does the Tur require erected doors here, but in Siman 275 he says that it suffices to lock them? Here, an owner is Makneh, but what is the source to distinguish?

Bach (DH ul'Fi): The Rosh distinguishes! After he proved that locking suffices, he asked from one who built on Hefker property, in which we require erecting doors. He brought the Rashbam, who says that erecting suffices by itself. The Rosh initially discussed Hefker. There, as long as the latter did not lock, he did not accomplish more than the first! Therefore, we must say that he locked them. Then the Rosh cites the Rashbam's opinion regarding a buyer. Here, regarding a buyer, no one argues with the Rashbam.

Prishah (275:10): The Maharshal asked why the Tur rules like Tosfos, that affixing and locking a door acquires. The Rosh concluded that affixing alone acquires! I proved (Prishah 192:2 DH b'Hamadas) that the Rosh says so only to acquire from another, but not to acquire Hefker.

Drishah (2): Even though the Rashbam requires erecting doors for Hefker, the Tur holds that the same applies to a buyer, for locking alone does not acquire for it is Mavri'ach Ari. Also, the Rashbam said so to explain 'No'al' in the Mishnah, which discusses also a buyer. Tosfos and the Rosh disagreed only with the Rashbam's opinion that locking does not acquire. They did not challenge the Rashbam's opinion that affixing a lock acquires, for they agree that it acquires for a buyer. It does not acquire Hefker, for there we need an action that truly improves the land. A buyer need make only a minor improvement..

Taz: We cannot distinguish, for Tosfos asked about a buyer from Hefker. The Gemara (57a) equates them. The Tur holds that just erecting doors does not acquire, for one can still enter. One must lock it, even without a frame, as long as it prevents entrance. In Siman 275 the Tur teaches that it must be locked, and here he teaches that one does not need a frame.

Gra (5): Here, the Shulchan Aruch and Rema omitted the opinion of Tosfos, the Rosh and all the Poskim, that locking alone acquires. The Tur says that the Rosh concludes like the Rashbam, that affixing a lock acquires. In Siman 275, the Tur and Rema rule like Tosfos! The Bach and SMA distinguish between acquiring Hefker and acquiring from a seller. This is difficult, for Tosfos and the Rosh asked the contradiction between these! The Rema is difficult, for he comments on the Shulchan Aruch, which is like the Rambam, who holds that it suffices to erect doors without locking! The Rambam and Shulchan Aruch (Sa'if 13) say that this is only for Hefker. There are three opinions about erecting doors. The Rambam holds that it helps only for Hefker. The Rosh holds that it does not help even at all. The Rashbam holds that it helps for Hefker and purchases.

Rema: The same applies if he made a lock for the door or erected doors. Some say that he acquired.

Shulchan Aruch (275:21): If Reuven built mansions on Hefker property, and Shimon put doors on them, Shimon acquires them. The form of the building is useful only once doors are set up.

Rema: Some say that the same applies if Reuven erected doors but did not lock them, and Shimon locked them.

Beis Yosef (DH u'Mah she'Chosav u'Chegon): The Rosh and Tur say that locking them acquires, but just erecting them does not. The Rambam connotes like the simple reading of the Gemara, that erecting alone acquires.