Built illegally on occupied Palestinian land. And to answer Nir's question: I believe King Hussein was the sovereign in the West Bank when it was captured by Israel in '67. However, in 1993 he relinquished all rights to the land in favor of a Palestinian state.
Dozens of Zionist Union activists call on Netanyahu to resign in Tel Aviv protest (Haaretz)
from the article: Palestinians in Ma'aleh Adumim employed by Israel but on Jordanian terms