Why did they rejoice over Yerushalayim?
Rashi: It was besieged.
Malbim: Tzor had a Bris with Yerushalayim from the days of David and Shalom. They did not rejoice over Yerushalayim's fall amidst hatred, only because they thought that it will benefit them.
What is "He'ach"?
Rashi: It is an expression of Simchah.
Why is Yerushalayim called "Dalsos ha'Amim"?
Rashi: The city in which all Nochrim entered for business. Malbim #1 - everyone entered through this door.
Malbim #2: It blocked in front of the nations that dwelled in the south, east and north. Their path to Tzor passed via Eretz Yisrael 1 and around it, like a closed door; one cannot enter until the door is opened. Now that the door was opened, [the nations] will turn to me.
This justifies calling Eretz Yisrael "Dalsos ha'Amim"! Perhaps Yerushalayim is called so, for it was the capitol. How did it block nations both in the north and in the south? Perhaps Tzor was close to Eretz Yisrael, all the good ports nearby were in Eretz Yisrael. (PF)
What doors of the nations were broken?
Rashi: The city in which all Nochrim entered for business ? it will break. The siege will not depart from it until it is captured.
What is the meaning of "Nasebah Elai"?
Rashi: From now, business of the nations will divert to me. Radak ? the root is Savev.
What will be filled?
Rashi: I (Tzor) will be filled from Yerushalayim, the destroyed city. Radak ? since it was besieged, they knew that it will not escape, and it will be destroyed. They considered it destroyed even before the Churban.
Malbim: "Mah Atem Li Tzor v'Tzidon v'Chol Gelilos Pelashes?; Asher Kaspi u'Zhavi Lekachtem u'Machamadai ha'Tovim Haveisem l'Heicheleichem; u'Vnei Yehudah? Mechartem" (Yo'el 4:4-6). They plundered and gathered money and slaves, and were filled from the Churban and booty.