What is "Mivchar Amakayich"?
Radak: This refers to Yerushalayim - the choice part of your land will be full of enemy chariots. It says Amakayich, because chariots do not go on mountains, only in valleys.
Radak citing Targum Yonasan: The best of your camps.
What is the meaning of "Shos Shasu ha'Sharah"?
Rashi: They placed a siege on the gates. We find in the war of Ben Hadad "va'Yasimu Al ha'Ir" (Melachim I, 20:12).
Radak #1: They set their faces to the gates of Yerushalayim to fight.
Radak #2: They fought; this is like "u'Fo Yashis bi'G'on Galecha" (Iyov 38:11).
Radak citing Targum Yonasan: They appointed [people] on the gates. Also "va'Yasimu Al ha'Ir" - the cavalry put their agenda and smashing tools to conquer the city.
Radak #3: They put siege Kelim to the gate.
Malbim: The cavalry put their faces to enter the gate. Only after this, and the valley being full of chariots, you put to heart to fight.
