Rashi: It was to strengthen the city. It was between the walls; the walls were double - "Chel v'Chomah" (Eichah 2:8, i.e. a low wall opposite the tall wall). The old pond was between ¹ the walls. Radak - it was near the old pond.

Malbim: They gathered the water lest the enemy have water to drink, and also, for them to have during the siege.

Rashi: Even though Chizkiyah (a great Tzadik) strengthened the walls and the city, he "trusted in Hashem Elokei Yisrael ¹ " (Melachim II, 18:5); you, in the days of Yehoyakim and Tzidkiyah, "did not look at [Yerushalayim's] Maker."

Radak: You did not look at the Creator, who made [Yerushalayim] and fixed it until now, and guarded it from the enemy, to repent and go in His ways. Had you done so, He would save you more than all the preparations you made!

Malbim: Why did you toil so much?! You should have looked with intellect - the enemy does not do all this. Hashem does. He made you fall in front of your enemies!

Rashi: From when He created the world, He considered Yerushalayim and the Beis ha'Mikdash; you did not think about this.

Radak: He formed Yerushalayim a long time before He made His Shechinah rest there, and chose it over all lands. You did not see Him to repent to Him.

Radak: He created it before the world was created. Chazal ¹ said, seven things were created before the world - Gan Eden, Torah, Tzadikim, Yisrael, Kisei ha'Kavod, Yerushalayim and Mashi'ach ben David. This is not like most people understand. Rather, they had potential to be created before the world was created, for they are the purpose of the creation.

Malbim: You should have seen that the enemy's victory is not natural; it is Hashem's deed. Further, you should have seen that Hashem decreed so a long time ago. Nevi'im prophesized so. You should have repented, to annul His decree!