1)
What is "Har ha'Mashchis"?
1.
Radak: It is Bavel. Also Yonason translates 'the city that destroys.' Even though it is a valley, it is a fortified city, as if it is on a mountain. Also "Haros ha'Gil'ad" (Amos 1:13) are fortified cities.
2)
What are "ha'Sela'im"?
1.
Radak: This is a metaphor for the tall towers in Bavel. Also Yonason translates 'the strong places' - i.e. even the strong tall places.
3)
What is "v'Gilgalticha"?
1.
Radak: Gilgul is [rolling] from a high place to a low place.
2.
Malbim: You will no longer dwell between the rocks. You will be a burnt mountain.