Why do Hakkas build tulou?

Hakkas are descendants of immigrants from the Central Plains who moved south in ancient times, so they have two architectural features: 1. Hakka people like to build rammed earth houses, which is a custom from the Central Plains. 2. Hakka people have a strong sense of clan, and like the whole clan to live in a big house, which is helpful for clan unity and theft prevention. The big house built by rammed earth is Hakka earth building. Earth buildings built by Hakkas can be seen everywhere in Hakka inhabited areas such as southern Jiangxi, western Fujian and northeastern Guangdong. Most of them are rectangular and long, and the scale of a building ranges from a few households to a dozen households. The source of Hakka earth buildings can be seen from the long rows of houses in Neolithic sites in the Yellow River valley. However, there are many round and semicircular earth buildings in several counties in southwest Fujian, which can accommodate hundreds of families. This is related to the frequent occurrence of local bandits in history. Some non-Hakkas in these places have also learned to build large earth buildings to prevent bandits.