John Gill Commentary 2 Chronicles 11:5

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

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.