Albert Barnes Commentary 2 Chronicles 11:5

"And Rehoboam dwelt in Jerusalem, and built cities for defence in Judah." — 2 Chronicles 11:5 (ASV)

Rehoboam was between two dangers: on the north, he might be attacked by Jeroboam; on the south, by Jeroboam’s ally, Egypt. The peril from this side was greater, and therefore, of the 15 fortified cities, all but three were on the southern or western frontier, where an attack from Egypt was most likely.