Walls were Probably often Strengthened with Plates of Iron or Brass
Source: TTT
Jeremiah 15:20
Then I will make you a wall to this people, a fortified wall of bronze; they will fight against you but will not overcome you, for I am with you to save and deliver you, declares the LORD.
Ezekiel 4:3
Then take an iron plate and set it up as an iron wall between yourself and the city. Turn your face toward it so that it is under siege, and besiege it. This will be a sign to the house of Israel.