There’s no doubt; they are redoubts
This aerial photograph, taken on October 31, 1953, shows a section of the York River bank at Yorktown, Virginia. Yorktown was the scene of the decisive victory by American and French forces against the British, effectively ending the Revolutionary War in 1781. Look closely in the upper left of the photograph to see outlines of the redoubts (small enclosed forts) built by the British as their main line of defense.
National Archives, Records of the Agricultural Stabilization and Conservation Service