Iolani Palace is the Hawaiian royalty's home.
Hammond Castle is located in Massachsetts.
Castello di Amorosa is built in the style of 12th century Tuscan castles. It is located in Napa Valley.
Bishop's Palace is located in Galveston, Texas.
Hearst Castle in California is 115 rooms. Has massive gardens, too.
Berkeley Castle in Berkeley Springs, West Virginia
Castillo de San Marcos, St. Augustine, Florida, actually built as a defensive fortification
There are several castles that were bought and moved brick by brick to the US by the wealthy in the late 1800s, one of which is in the Midwest. Wikipedia has a list of American castles that include those.