Complex sentence: During the 10th century stone began to be introduced into castles for the first time, initially in Normandy; in the 11th century stone castle, particularly stone keeps, began to spread through France and into England and Southern Wales as a consequence of the Norman invasions.
Split into simpler sentences: During the 10th century stone began to be introduced into castles for the first time, initially in Normandy. In the 11th century stone castle, particularly stone keeps, began to spread through France and into England and Southern Wales as a consequence of the Norman invasions.