Why does it say "Ki Heshcah"?
Radak: This shows that it is good to trust in Him. Because you, Beis Yisrael, trusted in Him, He lowered your enemies.
Malbim: You will not fear when others rise against you to harm you, for you saw how Hashem lowers them.
What is the meaning of "Yoshevei Marom"?
Rashi: [They dwell in] Tzur and other lands.
Radak: [He lowered your enemies, who dwelled in a high place, in a fortified city. He lowered every such place. Alternatively, it refers to one particular city that He lowered.
Malbim: They are strong due to their [high] place 1 .
Rashi (Koheles 5:1): Even if the weak is above and the strong is below, the latter fears the one above.
Why does it say "Kiryah Nisgavah"?
Malbim: It is intrinsically strong (not only due to its height).
Why does it repeat "Yashpilenah Yashpilah"?
Radak: The matter is repeated to strengthen it.
Malbim: Even though He already lowered it, He will lower it to the ground.
Why does it say "Ad Afar"?
Malbim: It will be like earth. There will be no mark at all that there was a city here.
