Ophel originally means a hill, citadel, or fortress. However, it specifically refers to the citadel right next to the city of Jerusalem, which was used as a defensive fortress.
2 Chr 27:3He built the upper gate of the house of the Lord and made many additions to the wall of Ophel.
Isa 32:14For the palace will be forsaken, the populous city deserted; the Ophel and the watchtower will become dens forever, a delight for wild donkeys, a pasture for flocks.
Later, during the return from Babylonian captivity, the Nethinim lived there.