National Historic Site
Established: October 12, 1976 (NPS 1980)
Annual Visitors: 3,652 (2005)
Official Site: https://www.nps.gov/euon/index.htm
Considering where this is located, it is very surprising that this is one of the least visited units in the National Park Service.
However... According to the NPS, "Self-guided tours are offered on 'Saturdays Without Reservations.' Due to the location of this park, all visitors are required to take a free National Park Service shuttle from the town of Danville to tour the historic home and grounds. Visitors are not permitted to drive their personal/private vehicles to the site."
Wikipedia adds, "The National Park Service does not publish the address of the property, but it is widely known that it is located near Kuss Road in Danville. A locked gate prevents unauthorized vehicles from reaching the site. The Site occupies 13 acres (5.3 ha) accessible via car only by private road, so advance reservations are required to visit."