How did the builders work on the wall while under threat of attack?

Answer: With a weapon in one hand

Those carrying materials worked with one hand and held a weapon with the other, and each builder wore a sword at his side.

“who were rebuilding the wall. The laborers who carried materials worked with one hand and held a weapon with the other. And each of the builders worked with his sword strapped at his side. But the trumpeter stayed beside me.”— Nehemiah 4:17-18 (BSB)