1)
What is the meaning of "ha'Omdos Al Tilam"?
1.
Rashi: The wall was not destroyed, unlike Yericho, and Ai, about which it says (8:28) "va'Ysimeha Tel Olam Shemamah."
2.
Radak, citing Targum Yonasan: It is in its strength. The wall did not fall, and it was not burned.
2)
Why did he burn only Chatzor?
1.
Rashi (from Bereishis Rabah 81:4): He had a tradition from Moshe (whom Hashem told) to do so.
2.
Radak #1: It was the capitol. The other would be afraid (when it was burned).
3.
Radak #2 (from Bereishis Rabah 81:4): [The Kena'anim] wanted to chase Yisrael; Hashem did not let them. They entered Chatzor.
