Albert Barnes Commentary 2 Chronicles 33:14

"Now after this he built an outer wall to the city of David, on the west side of Gihon, in the valley, even to the entrance at the fish gate; and he compassed Ophel about [with it], and raised it up to a very great height: and he put valiant captains in all the fortified cities of Judah." — 2 Chronicles 33:14 (ASV)

Rather, he built the outer wall of the city of David on the west of Gihon-in-the-valley. The wall intended seems to have been the one toward the northeast, which ran from the vicinity of the modern Damascus gate, across the valley of Gihon, to the “fish-gate” at the northeast corner of the “city of David.”

We may gather from this verse that, late in his reign, Manasseh revolted from the Assyrians and made preparations to resist them if they should attack him. Assyria began to decline in power around 647 B.C., and from that time its outlying provinces would naturally begin to fall away. Manasseh reigned until 642 B.C.