Caerphilly Castle comprises a set of eastern defences, protected by the Outer East Moat and the North Lake, and fortifications on the Central Island and the Western Island, both protected by the South Lake. The site is around in size, making it the second largest in Britain. It is constructed on a natural gravel bank in the local river basin, and the castle walls are built from Pennant sandstone. The castle's architecture is famous and historically significant. The castle introduced concentric castle defences to Britain, changing the future course of the country's military architecture, and also incorporated a huge gatehouse. The castle also featured a sophisticated network of moats and dams, considered by historian Allen Brown to be "the most elaborate water defences in all Britain".