Charles Spurgeon Commentary Nahum 3:14

"Draw thee water for the siege; strengthen thy fortresses; go into the clay, and tread the mortar; make strong the brickkiln." — Nahum 3:14 (ASV)

Fortify your strong holds: go into clay, and tread the mortar, make strong the brickkiln.

That was, to mend the walls whenever they were broken. They did this with great industry. "Do it," says God, "yet you shall not be able to stand."