The Zhencheng Building, a fine example of a 4-storey tulou. Tulou are fortified, adobe (earth) Hakka clan houses, many of them circular like this one, found only in the hills of Fujian Province in southern China. This unique architecture evolved as a means of defence when the Hakka migrated to this region beginning in the 13th century. A few thousand tulou remain today in use, of which perhaps one thousand are round, known as yuanlou. This building, in Hukeng Town, Yongding County, has 4 stories and 184 rooms in the outer ring, 32 rooms in the inner, with an ancestral hall in the centre.