Because of the many walls erected around the central tower, concentratic castles were difficult to conquer and penetrate. Moats, battlements, and gatehouses provided extra security. The overall effect was that a concentric castle was more defensible than an open ringwork fort. Concentric castles first appeared in Europe around 1150 and became popular throughout much of medieval times. They are found everywhere from England to Serbia. The best-known example is the Tower of London. Concentric castles use earthworks and standing stones instead of stone or brick for their defense. They are often found near large settlements or important resources such as mines or fishing boats. The word "concentric" comes from the Latin word meaning "around a center" or "in a circle". Although they look like circles, concentric castles are actually based on a hexagonal pattern with six towers each pointing in a different direction. The corners of this pattern are reinforced with extra defenses such as additional walling or moats. Often several concentric rings are built one over another. For example, the outer ring of the castle might be built using wood but the inner ring would be made out of stone. This allows people to rebuild areas of the castle destroyed in war or natural disasters without affecting the main structure or interior rooms.