Many northern European castles were originally built from earth and timber, but had their defences replaced later by stone. Early castles often exploited natural defences, lacking features such as towers and arrowslits and relying on a central keep. In the late 12th and early 13th centuries, a scientific approach to castle defence emerged. This led to the proliferation of towers, with an emphasis on flanking fire. Many new castles were polygonal or relied on concentric defence – several stages of defence within each other that could all function at the same time to maximise the castle's firepower. These changes in defence have been attributed to a mixture of castle technology from the Crusades, such as concentric fortification, and inspiration from earlier defences, such as Roman forts. Not all the elements of castle architecture were military in nature, so that devices such as moats evolved from their original purpose of defence into symbols of power. Some grand castles had long winding approaches intended to impress and dominate their landscape.