The castle of W Europe was a Norman creation, an outgrowth of the 10th- and 11th-century mound castle, which consisted of a great artificial mound of earth, the motte, surrounded by a dry ditch, or fosse, and surmounted by a wooden blockhouse and its encircling palisade. Until well into the 12th cent., the only English development was the occasional substitution of a massive masonry keep inside the palisade—a form typified in the Tower of London. As siegecraft (see siege) was evolved, provisions were made for an aggressive defense.
A castle that became the model for many English and Norman castles was the formidable castle built at Arques in Normandy by Henry I of England. A square donjon, or keep, was set against the strong outer walls of masonry; the entrance was protected by a double gate, two flanking round towers, and advanced earthworks. The place enclosed by the outer circuit of walls was usually divided into two courts, or baileys, by a palisade. Subterranean passages made detection of underground forays easy.