Most tulou buildings are circular or square shaped, but "there are other special shapes, such as oblongs -- it just depends on the landscape," according to architecture student Lin Weicheng, who has worked on tulou restoration.
All Chinese tulou buildings are found in Fujian province, mainly in its southwestern area.
Out of an estimated 3,000 tulous, 46 are recognized as World Heritage sites by UNESCO and are renovated, but many are in states of disrepair.
By usually being circular or square and only having one main entrance, tulous were well-designed for defense.
Larger tulous often had farm resources in the courtyards, so they could be self-sustainable during times of conflict.
The shape of tulous encourages families to interact a lot in the main courtyard. Sometimes entire Hakka clans would live in one tulou and share a collective name.
Facilities inside tulous often included shrines at which clan members could worship dead ancestors. Weddings were often held in them, too.
Tulous were often built at the foot of hills and close to rivers. "It just made it convenient for everyday life," Lin Weicheng says.
Lin Lusheng runs education projects in Fujian province to teach schoolchildren about the cultural history of tulous.
Many tulous are becoming empty as residents move to cities. "Young people don't live in tulous anymore," Lin Lusheng says.
Lin Lusheng, who grew up in a tulou, says: "It was common to see kids knocking on each other's doors to ask friends to eat and play."
Many of the earliest tulous were built by people from the ethnic group Hakka, who settled in Fujian having come from the north.
Tulous built by Hakka people had corridors running by all the different houses so it was easy for people to be connected -- those built by people from the Hokkien group do not have these corridors.
Lin Lusheng hopes to start a project that would allow families who used to live in tulous, but who have moved to cities, to send children back to their old tulous for school holidays.
Tulou walls were thick -- sometimes over a meter in width -- and built using a mixture of clays.