User blog comment:UnofficialGameExpert78/Age of Empires II HD: The American Tribes/@comment-34012722-20180911085850/@comment-30831233-20180911161138

North American architecture is based on cliff dwellings in the West, so the Castle would be pretty generic, featuring architectural features from them.

The Plains Castle would be a mound with a stone structure on top of it, with a wooden staircase leading up to the entrance.