1)
What are "Arim l'Matzor"?
1.
Rashi: They are small cities with water around them to strengthen (protect) them, like it says "v'Shasisi Mayim? Kol Ye'orei Matzor" (Melachim II, 19:24).
2)
Why did he build "Arim l'Matzor"?
1.
Rashi: Yisrael (the 10 tribes) rebelled against him.
2.
Malbim: He was afraid of Yaravam, who had many more subjects than Rechav'am.
