Rashi: It is Yerushalayim, which sits on the mountain of the plateau. All around the city is a flat field. Harari is like Aravi (3:2), one who dwells in Aravah (plain).

Radak: Yisrael are called Harari, for they served [idolatry] on the mountains. It is as if He calls to them - you are Harari. "Ba'Sadeh" refers to "Cheilcha" after this, even though the note on ba'Sadeh is Omed (a stop). You are Harari; in the field are your wealth and storehouses; I will give them to be plundered. I.e. the plunderers will take the spoils into the field to divide it there.

Malbim: You, My mountain, stand in the field. This is Har Tziyon; around it became like a field; the mountains around it were destroyed.

Rashi: It is your money.

Radak: No. It is as if it says v'Otzrosecha (with the prefix Vov). This is like "Shemesh Yare'ach" (Chabakuk 3:11), "Reuven Shimon" (Shemos 1:2) and similar verses.

Rashi: Your Bamos were made in sin, for the sake of idolatry, in all your borders.

Radak, Targum Yonasan: It is as if these words are reversed - b'Chatas Bamosecha, which are in all your borders. Why will all your money and all your storehouses be plundered? It is due to the sin of your Bamos. We often find words reversed like this, e.g. "Mekom Sham Kever" (Yechezkel 39:11) means Mekom Kever Sham. "U'Misped El Yod'ei Nehi" (Amos 5:16) means v'Yod'ei Nehi El Misped, and many similar verses.

Malbim: Also your Bamos will be plundered, due to the sin in all your borders.