Rehoboam was between two dangers: on the north he might be attacked by Jeroboam, on the south by Jeroboam’s ally, Egypt. From this side was the greater peril, and therefore out of the 15 cities fortified, all but three were on the southern or western frontier, where Egypt would be most likely to attack.
Related Commentaries of 2 Chronicles 11:5
2 Chronicles 11:5
5 And Rehoboam dwelt in Jerusalem, and built cities for defence in Judah.