Beaumaris Castle

Concentric design taken to its logical end. Begun in 1295 on a new site, Beaumaris let Edward I's master builder, James of St George, lay out rings of defenses with near-geometric symmetry: an outer wall with 16 towers and two gates, an inner ward with six more towers and two great gatehouses, and a water-filled moat with its own dock for supply ships. Anyone forcing the sea gate by the dock still faced 11 more obstacles, including a barbican, murder holes, portcullises and doors. The catch is that money ran out and the castle was never finished, and it saw little action apart from the English Civil War, so it shows the theory more than a tested result. Its symmetry makes the plan very easy to read on a visit.