Albert Barnes Commentary 2 Chronicles 8:5

"Also he built Beth-horon the upper, and Beth-horon the nether, fortified cities, with walls, gates, and bars;" — 2 Chronicles 8:5 (ASV)

Built - "Rebuilt," or "repaired" (as in 2 Chronicles 8:2). The two Beth-horons were both ancient cities (see the note on Joshua 10:10).