John Gill Commentary 2 Chronicles 17:12

"And Jehoshaphat waxed great exceedingly; and he built in Judah castles and cities of store." — 2 Chronicles 17:12 (ASV)

And Jehoshaphat became very great exceedingly
In wealth and riches, power and authority, in his forces and fortifications:

and he built in Judah castles, and store cities ;
castles for the defence of his kingdom, and store cities to lay up corn, and wine, and all provisions, in case of an invasion, or against a time of war.