Where is "Al Tzafon"?
Rashi: Ashur, which is in the north, by Bavel.
Radak: It is Bavel, which is northeast of Eretz Yisrael. It was mentioned, because Yisrael were exiled to there, and [Bavel] did not have proper mercy on them - "Lo Samt Lahem Rachamim" (Yeshayah 47:6).
Why will He destroy Ashur?
Radak: The 10 tribes were exiled to there 1 .
Is this a reason to destroy Ashir? The 10 tribes were exiled to Lachlach v'Chavor. In Yeshayah 6:12, Radak said that these are far away, at the end of the land. Perhaps the word "Sham" in Radak here is a mistake; rather, he says that Ashir I them. (PF)
Why does it mention Ninveh?
Rashi: It is the capitol of Ashur. Malbim - Nebuchadnetzar destroyed it in the first year of his reign.