Casemate

This is a casemate, or armored fortress, at Fort Monroe. Fort Monroe is on Old Point Comfort in Hampton, Virginia. It was an active military service from 1607-2011 when it became a US National Monument (bureaucrat-speak for park).

In it's active time, it was the only US military fort with a moat...which you are looking at in this picture.

During the US Civil War, it was always held by the union, despite it's location in confederate Virginia. After the war, it held confederate president Jefferson Davis for a time.