A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
2 Chronicles 27:1
ch2 27:1JOTHAM, REIGNING WELL, PROSPERS. (Ch2 27:1-4)
Jotham was twenty and five years old--(See on Kg2 15:32-35).
His mother's name . . . Jerushah, the daughter of Zadok--or descendant of the famous priest of that name [Sa2 8:17].
2 Chronicles 27:2
ch2 27:2he did that which was right--The general rectitude of his government is described by representing it as conducted on the excellent principles which had guided the early part of his father's reign.
the people did yet corruptly--(See Kg2 15:35); but the description here is more emphatic, that though Jotham did much to promote the good of his kingdom and aimed at a thorough reformation in religion, the widespread and inveterate wickedness of the people frustrated all his laudable efforts.
2 Chronicles 27:3
ch2 27:3He 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 Kg2 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 TyropÅon, called "the lower city" [JOSEPHUS]. He
built much--having the same desire as his father to secure the defense of Jerusalem in every direction.
2 Chronicles 27:4
ch2 27:4in the mountains of Judah, and in the forests he built castles and towers--that is, in the elevated and wooded spots where fortified cities could not be placed, he erected castles and towers.
2 Chronicles 27:5
ch2 27:5HE SUBDUES THE AMMONITES. (Ch2 27:5-9)
He fought also with the king of the Ammonites--This invasion he not only repelled, but, pursuing the Ammonites into their own territory, he imposed on them a yearly tribute, which, for two years, they paid. But when Rezin, king of Syria, and Pekah, king of Israel, combined to attack the kingdom of Judah, they took the opportunity of revolting, and Jotham was too distracted by other matters to attempt the reconquest (see on Kg2 15:37).