Iron

Then they lifted him up and flew away with him to the Witch’s castle, where he was placed in a small yard with a high iron fence around it, so that he could not escape.

Within there was a small corridor, which ended in a very massive iron gate.

In the first place, I may tell you that Moriarty rules with a rod of iron over his people.

She placed a bar of iron in the middle of the kitchen floor, and then by her magic arts made the iron invisible to human eyes.