Rashi: It is Yerushalayim - the Gei (canyon) where most Nevu'os were. Radak -Yerushalayim is a mountain! It is called a valley due to the evil of its residents. They lowered it from a mountain to a valley. Therefore, it is called a valley, and not a Har.

Malbim: It is about Yerushalayim, which fell to Nebuchadnetzar.

Rashi: When the army came against them, they ascended to their roofs to see the army and fight.

Rashi, from Ta'anis 29a: [At the time of the Churban,] Kohanim ascended to the roof of the Heichal, holding the keys to the Azarah [and said that they were not faithful Gizbarim. They threw the keys up, and the form of a hand caught them. They jumped into the fire.]

Rashi, from Eichah Rabasi, Pesichah 24: They were haughty.

Radak: Normally, when the enemy enters the city with force, the residents ascend to the roofs and towers to take refuge there, or to fight if they can. The Navi says to Yerushalayim, why do you do so? Yesterday you were a full, noisy city - why do you now enter houses and ascend to roofs? It is due to your sin!

Malbim: This Nevu'ah depicts that a man sees what occurs in Yerushalayim from the beginning of the siege until the city was captured. He sees strange things that show that it was not captured naturally; rather, it was Hashem's hand to confound and destroy them. He is astounded, at the beginning of the siege, why did everyone ascend to the roofs to fight from there? Normally, Giborim go out to fight in the gate. Only women and the weak ascend to the roofs. Why did all of you, even the strong, ascend?