From Wikipedia:

Hakka architecture is a building style in southern China unique to the Hakka people. They are typically designed for defensive purposes and consist of one entrance and no windows at ground level.

The Hakka were originally immigrants from northern China who settled in the southern provinces. From the 17th century onwards, population pressures drove them more and more into conflict with their neighbours (called punti in Cantonese). As rivalry for resources turned to armed warfare, the Hakka began building communal living structures designed to be easily defensible. These houses, sometimes called tulou, were often round in shape and internally divided into many compartments for food storage, living quarters, ancestral temple, armoury etc. The largest houses covered over 40,000 m² and it is not unusual to find surviving houses of over 10,000 m².

Source: Wikipedia
Photos: Iwan
Via: Arch Daily