Rashi: When [Yisrael] will do so (repent), the fortified city of Esav (some texts - Yishmael) will be alone.

Radak #1: Now that they serve idolatry, the city that was fortified, in their sins it is desolate without residents.

Radak #2: The fortified city, in the future it will be desolate due to their sins. This refers to Yerushalayim or Shomron. The [10] tribes were not yet exiled when this Nevu'ah was said - "Hoy Ateres Ge'us Shikorei Efrayim" (28:1).

Radak (11, based on Targum Yonasan): When Yisrael will remove the idolatry, the fortified city of Nochrim will be alone (desolate).

Malbim: This refers to "hak'Makas Makehu Hikahu" (7). There is a great difference between Hashem striking Nochrim, with cruelty until there was no remnant, and striking Yisrael, in order to get them to repent. When Hashem strikes fortified cities of Nochrim, they become desolate.

Radak #1: What used to be a dwelling for man, it is now a place for animals to graze.

Radak #2: "Naveh" refers to the Beis ha'Malchus, or the Beis ha'Mikdash, which will be destroyed due to their sins. It will be sent and abandoned like a Midbar.

Malbim: A place of gardens and orchards [of Nochrim] will become wild and Hefker.

Rashi: Efrayim will inherit it. It is called Egel - "k'Egel Lo Lumad" (Yirmeyah 31:17).

Radak #1: Calves and other animals will graze there.

Radak #2: This is a Mashal. My father said, it is a metaphor for Melech Mitzrayim; Egypt is called "Eglah Yefefiyah Mitzrayim" (Yirmeyah 46:20). This is when he took Yeho'achaz (Melachim II, 23:34).

Radak #1: Keneses Yisrael is compared to a vineyard, so it says that the calf will graze and finish off the branches, like it said in the first Nevu'ah "Haser Mesukaso v'Hayah Leva'er" (5:5).

Radak citing Ibn Ezra: Se'ifeha is a metaphor for open cities.

Radak #2: If Egel is a metaphor for Pharaoh, Se'ifeha is a metaphor for the [tax of] silver and gold that he imposed on the land.

Malbim: Animals will graze there constantly, to the point that the branches will be finished off.