Turku's most famous monuments are the ancient castles and cathedrals left over from the Middle Ages. The ancient castles and cathedrals are situated on the banks of the Ola River. The old castles are located at the mouth of the lower reaches of the Ola River. The cathedral is located in the downtown area of the upper reaches, far from the old castle. Turku Cathedral is the main church of Lutheranism in Finland and the most famous medieval cathedral in Finland. The cathedral was built in 1229. Pure naturalism has been proved here again. The cathedral is made of primitive rocks. After many times of expansion, it has been continuously heightened and thickened to the present scale. Looking from the outside, the tall exterior wall has no whitewash, showing the mottled traces of perennial wind and rain erosion.