What is the meaning of "Mei Matzor Sha'avi Lach"?
Rashi: When enemies come to besiege a city, people of the city gather much water in it in barrels to drink during the siege, if they do not have pits [of water].
Radak: Do all that you can - it will not help you. Malbim - the verse mocks their efforts to strengthen their fortresses and take refuge in them. One who wants to take refuge in a fortress needs to prepare an abundant amount of water. You do not have this - rather, draw water for the siege, a small amount in Kelim. Also, it will not last long; it will spoil.
What do we learn from "Chazki Mivtzarayich"?
Malbim: Your fortresses are not strong.
Why does it say "Bo'i va'Tit v'Rimsi va'Chomer"?
Rashi: Mix the water in mud to make bricks to strengthen weaknesses in the walls. Radak - v'Rimsi va'Chamor is a repetition.
Malbim: Tit (mud) is not proper for bricks; they crumble. Rather, they are made from Chemar mortar. Since you have no Chemar, trample the mud to make it strong like Chemar
Why should they hold the Malben?
Rashi: Hold in your hands the frame to make bricks in it.
Radak: Malben is the furnace in which they make bricks.
Malbim: Since you use Tit, and not Chemar, you must hold the mold well lest the brick crumble.
