Chapter 11

Rehoboam, because of Israel's rebellion, raised an army of 180,000, but God warned them not to attack.  Shemaiah is the prophet mentioned, both here and in 1Kgs, as conveying the warning not to attack.  So he built cities for the defense of Judah.  Bethlehem is one of those cities, along with Etam, Tekoa, and so on.  These were all fortified cities.  It may be that some were already there, but Rehoboam built them up and fortified them for defense.  He stored food and wine in each, and put commanders there.  Also shields and spears were stored in each of these cities.  "...So he held Judah and Benjamin."  Still, Jerusalem is very near the border with Israel.  There just wasn't much real estate there to make a defense.  Jerusalem would be hard to hold against an invading army it seems to me.

Verses 13-17 are information not in 1K.  It says that the Levites and priests left Israel and came to Jerusalem.  They left their homes and lands permanently, and all those in Israel who wanted to worship God came with them.  This was because Jeroboam and his sons "fired" the priests and Levites and appointed their own.  Many refused to worship the golden calves Jeroboam made, and left their homes to avoid doing so.  It seems, therefore, that God's retribution against the north was not just about their King's transgressions.  The people who stayed home and worshiped those calves put home and possessions ahead of God.  Then it says that for three years these people from the north strengthened the Kingdom of Judah, and made it secure, because for those three years "...they walked...in the way of David and Solomon."  So even though God had condemned all of Israel - the northern tribes - the ones who loved and worshiped God, who were faithful to him - have this chance to relocate, to get out of the God-abandoned north, and come to reside in the south.  There may have been quite a lot of these because it wasn't just the Levites, it was the people too, who just would not worship calves and goats.  Their numbers were sufficient to strengthen the southern kingdom - perhaps they supplied much of the labor to build the fortified cities and then to populate and guard them. 

Rehoboam marries.  The wives mentioned are distant relatives of his.  Maacah, one of Absalom's daughters, is his favorite of 18 wives and 60 concubines.  Rehoboam has 28 sons 60 daughters.  He chooses Abijah son of Maacah as chief prince, intending to make him King.  Both Rehoboam and Jeroboam have a son named Abijah.  Jeroboam's son dies, though God found some favor in him, as part of God's punishment of Jeroboam for the calves.  Rehoboam also  "distributes" his sons around Judah in the fortified cities.