What will remain in the city?
Radak: Joy departed; only desolation remains. "B'Ir" refers to every city of Nochrim.
What do we learn from "u'She'iyah Yukas Sha'ar"?
Bava Kama 21a: If one lived in another's yard without the owner's knowledge, he need not pay, for "u'She'iyah 1 Yukas Sha'ar." (The owner benefits if someone is there!)
Rashi: Due to desolation, that no one inhabits the houses, Mazikin (damaging angels) will strike the gates.
Radak: No one leaves or enters [the gate]. She'iyah seized there, like "Im Sha'u Arim me'Ein Yoshev" (6:11), and all this great punishment will be in the future, with Gog and Magog and other Nochrim. These three verses at the end of the Parashah (13-15) prove so. Only a small amount of the Nochrim will remain - "v'Nish'ar Enosh Miz'ar", "k'Nokef Zayis" (6, 13).
Malbim: Many used to gather in the gate of the city, and the Zekenim sat there - "Zekenim mi'Sha'ar Shavasu" (Eichah 5:14). Now it will be desolate.
21a: Mar bar Rav Ashi saw it. It gored like an ox. (Rashi - it is a demon. Ramah, in Nimukei Yosef - it is mold, and it damaged like a goring ox.
