In a post to the Armor Archive, Richard Blackmoore said,
"The 'castle' was Hempstead House, the mansion at the Sands Point Preserve in Port Washington, NY. A Gold Coast era mansion on Long Island's North Shore, where MSR gets to hold events as part of their putting on an annual Medieval Fair. The area where they were fighting is a stone fence bordered yard where they joust during the fair (I used to do that there, lots of fun, nice scenery). The same preserve has another mansion, Falaise, where Lindberg wrote his famous book on flying. The estates belonged to the Goulds and the Guggenheims and is now run by Nassau County as a nature preserve."
Richard Blackmoore, a former member of MSR, recognized the facility as well as some of the heraldry and trappings from the event.
To learn more about Hempstead House, visit the "original article" link.