[maquiladora]

In FOTR we see him in the Ring-world, he isnt actually wearing that crown in reality, only through Frodo's eyes. This helmet/crown seems to be the "real" one.

[Vast Buffalo Wing Conspiracy]

Ah. That does make sense. Looks like they're modeling him on the Alan Lee painting, of the Witch King/Eowyn fight, which shows him wearing a "helm" like that. I think the book describes it more like a crown, but my memory is a bit unclear on that.