Boldt Castle Heart Island, NY
Chateau LaRoche Loveland, OH
Coral Castle Homestead, FL
Gillette Castle State Park East Haddam, CT
Hammond Castle Gloucester, MA
Hearst Castle San Simeon, CA
Manresa Castle Port Townsend, WA
Miramont Castle Manitou Springs, CO
Mystery Castle Phoenix, AZ

If you're interested in seeing more Castles of the United States click on the following link:

The gentlemen who maintains this website does a fantastic job 
and presently has over 200 castles listed.
Be sure and check out the castle he is building in West Virginia.