1)
What is "ha'Milo"?
1.
Rashi: It is a low wall around the fortress. They make a mound of earth sloping down on all sides; David built houses on it.
2.
Radak: It is a wide place near the wall where the nation would gather. This is like "Kir'u Mal'u He'asfu v'Navo'ah" (Yirmeyah 4:5). He built [not there, rather,] inside of it (closer to the Metzudah); Yo'av built the rest of the city (Divrei ha'Yamim I, 11:8). Malbim - Yaravam rebuked Shlomo for building the Milo (Melachim I, 11:27, for this removed the place for the nation to gather).
