Originally, a castle was a fortress built to protect strategic locations from enemy attack or to serve as a military base for invading armies. The earliest castles in Europe were constructed of earthwork and timber. Dating as far back as the 9th century, these early structures were often built over ancient Roman foundations.
Over the next three centuries, wooden forts evolved into imposing stone walls. High parapets, or battlements, had narrow openings (embrasures) for shooting. By the 13th century, lofty stone towers were popping up across Europe.
People seeking protection from invading armies built villages around established castles. Local nobility took the safest residences for themselves - inside the castle walls. Castles became homes, and also served as important political centers.
As Europe moved into the Renaissance, the role of castles became divided. Some were used as military fortresses, and were controlled by a monarch. Others were unfortified palaces, mansions, or manor homes and served no military function.
Although Europe and Great Britain are famous for their castles, imposing fortresses and grand palaces have played an important role in most countries around the world. Japan is home to many impressive castles. Even the United States claims hundreds of modern "castles" built by wealthy businessmen. Some gilded age homes resemble medieval castles.
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A castle built as a military stronghold may be called a fort, fortress, stronghold, or stronghouse. A castle built as a home for nobility is a palace. In France, a castle built for nobility may be called a chateau(the plural is chateaux).