Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
This short chapter runs parallel with 2 Kings (marginal reference), and is taken mainly from the same source or sources which it amplifies.
Ophel was the name given to the long, narrowish, rounded spur or promontory, which intervenes between the central valley of Jerusalem (the Tyropoeon) and the Kidron, or valley of Jehoshaphat. The anxiety of Uzziah and Jotham to fortify their territory indicates a fear of external attack, which at this time was probably felt mainly in connection with Samaria and Syria (Kg2 15:37 note). The faithless trust put in fortifications was rebuked by the prophets of the time Hos 8:14; Isa 2:15.
The Ammonites, who had submitted to Uzziah Ch2 26:8, revolted against Jotham. This revolt he firmly repressed; and, to punish it, he exacted a high rate of tribute for the three years following the termination of the war. The productiveness of the Ammonite country in grain, which is here indicated, has been remarked upon as extraordinary by modern travelers.