Rehoboam lived in Jerusalem and built up towns for defense in Judah:
References for 2 Chronicles 11:5
Bethlehem, Etam, Tekoa,
Beth Zur, Soko, Adullam,
Gath, Mareshah, Ziph,
Adoraim, Lachish, Azekah,
Zorah, Aijalon and Hebron. These were fortified cities in Judah and Benjamin.
References for 2 Chronicles 11:10
He strengthened their defenses and put commanders in them, with supplies of food, olive oil and wine.
He put shields and spears in all the cities, and made them very strong. So Judah and Benjamin were his.
The priests and Levites from all their districts throughout Israel sided with him.
The Levites even abandoned their pasturelands and property and came to Judah and Jerusalem, because Jeroboam and his sons had rejected them as priests of the LORD
References for 2 Chronicles 11:14
when he appointed his own priests for the high places and for the goat and calf idols he had made.
References for 2 Chronicles 11:15
Those from every tribe of Israel who set their hearts on seeking the LORD, the God of Israel, followed the Levites to Jerusalem to offer sacrifices to the LORD, the God of their ancestors.
References for 2 Chronicles 11:16
They strengthened the kingdom of Judah and supported Rehoboam son of Solomon three years, following the ways of David and Solomon during this time.
References for 2 Chronicles 11:17