Once the American Civil War began in 1861, Confederate forces used the Windsor mansion cupola as an observation platform and signal station. In the spring of 1863, as part of his Vicksburg campaign, Ulysses S. Grant and 17,000 Union troops landed at the port of Bruinsburg and took control of Windsor mansion. Following the Battle of Port Gibson, the mansion was used by Union troops as a hospital and as an observation station. The Daniell family was allowed to live on the third floor of the mansion during the Union occupation.