Many ancient standing stone monuments were erected during the prehistoric period, amongst the best known are Stonehenge, Devil's Arrows, Rudston Monolith and Castlerigg. With the introduction of Ancient Roman architecture there was a development of basilicas, baths, amphitheatres, triumphal arches, villas, Roman temples, Roman roads, Roman forts, stockades and aqueducts. It was the Romans who founded the first cities and towns such as London, Bath, York, Chester and St Albans. Perhaps the best known example is Hadrian's Wall stretching right across northern England. Another well-preserved example is the Roman Baths at Bath, Somerset. Early Medieval architecture's secular buildings were simple constructions mainly using timber with thatch for roofing. Ecclesiastical architecture ranged from a synthesis of Hiberno–Saxon monasticism, to Early Christian basilica and architecture characterised by pilaster-strips, blank arcading, baluster shafts and triangular headed openings. After the Norman conquest in 1066 various Castles in England were created so law lords could uphold their authority and in the north to protect from invasion. Some of the best known medieval castles include the Tower of London, Warwick Castle, Durham Castle and Windsor Castle amongst others.