Smith's Bible Dictionary
O'phel. (hill). A part of ancient Jerusalem. Ophel was the swelling declivity, by which the mount of the Temple slopes, on its southern side into the valley of Hinnom - a long, narrowish rounded spur or promontory, which intervenes between the mouth of the central valley of Jerusalem, (the Tyropoeon), and the Kidron, or valley of Jehoshaphat.
Halfway down it, on its eastern face is the, "Fount of the Virgin," so called; and at its foot the lower outlet of the same spring - the Pool of Siloam. In 2 Chronicles 27:3, Jotham is said to have built much, "on the wall of Ophel." Manasseh, among his other defensive works, "compassed about Ophel." Ibid. 2 Chronicles 33:14. It appears to have been near the "water-gate," Nehemiah 3:26, and the "great tower that lieth out." Nehemiah 3:27. It was evidently the residence of the Levites. Nehemiah 11:21.
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Smith, William, Dr. Entry for 'Ophel'. Smith's Bible Dictionary. https://www.studylight.org/dictionaries/eng/sbd/o/ophel.html. 1901.