The iron giant is the guardian of the entrance to the Nome Kingdom. He is a mechanical man who wields a massive hammer, bringing it down forcefully with a steady rhythm.
Like Tik-Tok, he was created by Smith & Tinker, but not given any speaking or thinking apparatus.
Only the Nome King held the creatures key, and as such, was the only one who could deactivate the thing. That was, until Dorothy, with the help of her friends, defeated the Nome King, whereupon she used his magic belt to still the giant once and for all.
There he still stands, just outside the entrance to the Nome Kingdom, hammer poised, ready to strike the earth once more.