The moat, which was originally supplied by the Thames but has been drained since 1843, surrounds the outer curtain. Outside the moat, the wall features gun embrasures; opposite these, contemporary artillery pieces are fired ceremonially on state occasions. The entire building complex spans 18 acres (7 hectares). The word "curtain" comes from the old English word "curtaen," which means a fence or hedge. Thus the term "curtain wall" refers to an exterior wall that encloses a property or institution. The phrase "Tower of London" is used interchangeably with "London Castle." The castle stands on a mound of earth within the walls of the City of London. First built in 1066, the current structure is mainly 12th century. It was renovated in the 15th century after suffering damage during the Battle of Barnet in 14AD when the future king Edward IV defeated the forces of his uncle, Richard III, who had claimed the throne. The Tower has been used for imprisonment, as a treasury, and as a place of execution. Today it is home to several royal families' treasures, including the Crown Jewels. The tower also contains three museums: the Crown Jewels, the Medieval Palace, and the White Tower. In conclusion, the outer curtain of the Tower of London is 16 feet (5 m) thick. It was built between 1171 and 1220.