Jamieson, Fausset and Brown on 2 Chronicles 27:3

3. He built the high gate of the house of the Lord--situated on the north--that portion of the temple hill which was high compared with the southern part--hence "the higher," or upper gate (see on 2Ki 15:35). He built, that is, repaired or embellished.

and on the wall of Ophel--Hebrew, "the Ophel," that is, the mound, or eminence on the southeastern slope of the temple mount, a ridge lying between the valleys Kedron and Tyropoeon, called "the lower city" [Josephus]. He

built much--having the same desire as his father to secure the defense of Jerusalem in every direction.