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.

On April 1, 1987, flood waters swirled eight feet high around the structure and dismantled it completely. Crews in boats recovered the log timbers scattered for many miles downriver and the blockhouse was reassembled the following year. Today, the fort bears the designation as a National Historic Landmark.