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.

And built cities for defense in Judah - He was obliged to strengthen his frontiers against the encroachments of the men of Israel; and Jeroboam did the same thing on his part to prevent the inroads of Judah. See 1-Kings 12:25.

And Rehoboam dwelt in Jerusalem, and (c) built cities for defence in Judah.
(c) Or, repaired them and made them strong to be more able to resist Jeroboam.

And Rehoboam dwelt in Jerusalem,.... The metropolis of Judah, and the capital city of his kingdom:
and built cities for defence in Judah; that is, rebuilt, enlarged, and fortified them; for otherwise they were built before, though neglected before the revolt of the ten tribes; but now it became necessary to make them more capacious and strong, to protect his people, and defend himself against Israel; for though he was forbid to act offensively, and therefore contented himself to abide in Jerusalem, and not go forth to war; yet he might lawfully put himself into a condition of defence.

built cities for defence in Judah--This is evidently used as the name of the southern kingdom. Rehoboam, having now a bitter enemy in Israel, deemed it prudent to lose no time in fortifying several cities that lay along the frontier of his kingdom. Jeroboam, on his side, took a similar precaution (1-Kings 12:25). Of the fifteen cities named, Aijalon, now Yalo, and Zorah, now Surah, between Jerusalem and Jabneh [ROBINSON], lay within the province of Benjamin. Gath, though a Philistine city, had been subject to Solomon. And Etham, which was on the border of Simeon, now incorporated with the kingdom of Israel, was fortified to repel danger from that quarter. These fortresses Rehoboam placed under able commanders and stocked them with provisions and military stores, sufficient, if necessary, to stand a siege. In the crippled state of his kingdom, he seems to have been afraid lest it might be made the prey of some powerful neighbors.

Rehoboam's measures for the fortifying of his kingdom. - To defend his kingdom against hostile attacks, Rehoboam built cities for defence in Judah. The sing. למצור is used, because the building of cities served for the defence of the kingdom. Judah is the name of the kingdom, for the fifteen fenced cities enumerated in the following verses were situated in the tribal domains of both Benjamin and Judah.

Built - Repaired, enlarged, and fortified them. They were built before.