See 1 Kings 4:26 note. Until the time of Solomon, war-chariots had not been in use among the Jews, except to a very small extent 1 Chronicles 18:4. Hence, it was necessary for him to put himself on an equality in this respect with neighboring powers.
Cities for chariots - They were probably fortresses upon the borders of his territory, in which he maintained the standing army necessary for the support of his dominion.