The fort was built of cottonwood lumber cut from the riverbanks. It was triangular in shape, with high walls on all sides and a gate facing the riverbank. Storage rooms provided a safe place to keep their supplies. The Corps of Discovery started the fort on November 2, 1804, and remained in the area until April 7, 1805. They built the fort slightly down river from the five villages of the Mandan and Hidatsa nations.