Rashi (Shmuel II, 5:9): It is a low wall around the fortress. They make a mound of earth sloping down on all sides; David built houses on it.

Radak: It is a wide place near the wall where the nation would gather ¹ . This is like "Kir'u Mal'u He'asfu v'Navo'ah" (Yirmeyah 4:5).

Vilna Gaon: When they build a fortress for a city, it has a round wall of rocks around it. To protect the wall from Kelim to destroy walls (e.g. catapults), they make another wall with corners around it, and fill the wall with much earth. This is called Milo.

Radak: David did not build the Milo, rather, inside (closer to the Metzudah) until ha'Saviv (around the tower). Shlomo built the Milo. Yo'av built the rest of the city.

Malbim: Shmuel II 5:9 teaches that David built "Saviv Min ha'Metzudah va'Baysah" within the wall of the Metzudah. Here it teaches that afterwards, he built outside the Metzudah, until the Milo. Shlomo built the Milo, a wall around it and a wall around Yerushalayim. David himself oversaw Ir David. Yo'av was Rosh over Yerushalayim, like David promised him (for striking Yevusi).

Rashi: He built and strengthened cracks in the wall. We find [an expression of Chiyus] like this ? "ha'Yichyu Es ha'Avanim me'Aremos he'Afar" (Nechemyah 3:34).

Radak: He built the rest of the city, which was destroyed. An expression of Chiyus regarding building is a metaphor. The same applies to "ha'Yichyu Es ha'Avanim me'Aremos he'Afar" (Nechemyah 3:34) and "va'Yrapei Es Mizbach Hashem he'Harus" (Melachim I, 18:30).

Vilna Gaon: He built the houses of Yerushalayim, for Yehoshua burned it. This is like "ha'Yichyu Es ha'Avanim me'Aremos he'Afar v'Hemah Serufos" (Nechemyah 3:34).

Malbim: If needed, he added residents or breathed life into it, to make it a busting, happy city, or oversaw to ensure that there is livelihood, food and sustenance.