The oldest blockhouse in the United States is all that remains of Fort Halifax at the confluence of the Kennebec and Sebasticook Rivers in Winslow, Maine. English settlers built the fort in 1754 to protect colonial settlements along the Kennebec and it served as a garrison for troops from 1754-1766. One mile south of the Winslow-Waterville bridge on U.S. Route 201. This is a state historical site and park. The building opens once a year on the 4th of July.