Inside the centuries-old Zhencheng Lou, one of the oldest buildings in the area.

Residents of Dongcheng Lou sit at the entrance of the building.

"The buildings are mainly four or five stories high. The first floor serves as the kitchen, the second is used for grain storage and the upper floors act as living areas," Xu said. "Because of their defensive function, only rooms on the third floor and higher have windows - and they are very small. With enough food, the residents could survive a long time under attack."

With their unique appearance, architectural techniques and group lifestyles, the tulou have gradually attracted global attention. In 2008, UNESCO put 46 on its World Heritage List.

The sudden fame changed Yongding county and the lives of the residents.

Roads and other infrastructure were built to make the tulou more accessible, leading to an influx of visitors.

"Tourism changed our village, and most of the residents now work in tourism-related businesses, such as running home inns, restaurants, selling local products or souvenirs, or working as tour guides. There are more than 100 cars in our village," Lin said. "One in four families owns one."

In 2008, Lin Bingxiang, 32, the fourth-generation owner of Yucheng Lou, quit his job at a large logistics company in Xiamen and returned home. He poured all the money he had earned during the previous few years into opening a restaurant and turning his tulou into a small home inn.

The 153-year-old Yucheng Lou was once home to 600 people, but now two large families comprising around 20 people live there.

"If I hadn't quit, I might be a manager in the company now," said Lin Bingxiang. "But I don't regret it at all because I have a very good business here and, more important, I can live with my parents. It's my ideal life."

Jiang Jaolin, 29, a resident of Hongkeng village who once worked as an accountant in Xiamen, also returned and now works as a tour guide.

"I can make a living from home and take care of my little son during work breaks," Jiang said. "We can sell all the things we grow, even the pickles my mother makes. Life is much easier than before, and I enjoy a better environment."

Most tourists arrive in the morning and leave before 3 pm. Those seeking a little peace and quiet stay overnight, and are usually polite and thoughtful.

"To tell you the truth, our life isn't disturbed too much," Jiang said.

Former president Hu Jintao visited Zhencheng Lou during the 2010 Spring Festival and commented that the Fujian tulou are not only a World Heritage Site, but also a treasure of traditional Chinese culture.

Hu's visit triggered a tourism boom in Yongding and tourist numbers have increased sharply since.

Lucy Ullrich, an urban designer for Tauranga City Council in New Zealand, is one of them.

Ullrich saw photographs of the tulou in a magazine several years ago and never forgot the fascinating images, so when she was invited to join a conference in Shanghai, she saw a chance to visit.

Instead of staying in hotels, she chose tulou accommodation, which cost about 100 yuan ($16) a night, and spent most of the time observing the lives of the local people and chatting with the people in broken Chinese.

"Although the sun was scorching outside, I didn't need to turn on the fans to keep cool in the room," she said. "The design is perfect for saving energy and making people comfortable. I may incorporate some tulou factors in my future designs. The nights here are so quiet and the local people are very friendly. I'll bring my family next time."

More than 4 million tourists from China and overseas visited the county in 2012, a year-on-year increase of 17 percent, and the revenue from tourism soared to 2.85 billion yuan, according to Yongding County Tourism Bureau.

Protecting the past

To keep his tulou in prime condition, Lin Qinsheng, the owner of Fuyu Lou, has rented out rooms for the past eight years, charging 500 yuan a month for each room.

"The tulou can survive hundreds of years if people live in them," he said. "Otherwise, they are quickly ruined by insects and lack of maintenance."

Protection is always on the top of the agenda for local governments trying to develop the potential of these astonishing structures.

"To protect the tulou properly we founded the first county-level cultural relics bureau in Fujian to improve the protection work and draw up a five-year maintenance plan," said Mao Gaoliang, Party secretary of Yongding county.

No buildings can be constructed on a site once it has gained world heritage status, so the county has bought a series of facilities outside the village and offers lower rents to encourage local people to move their businesses out of the heritage site.

Lin Rigeng is the best-known guide in Yongding and has seen great benefits from tulou-based tourism. The increasing importance of Zhencheng Lou has given him plenty of chances to act as a guide for government officials, foreign dignitaries, and famous actors and singers, as evidenced by the photos that cover almost half a wall of one of his rooms.

"Without tourism, my family wouldn't have such a good life and the young people wouldn't return. Even my son quit his job as a teacher and brought his family back to help me manage the business," he said. "I am a third-generation tulou owner and I'm protecting our heritage so that more people will have the chance to see the architectural treasures of the Hakka people."

Sun Li contributed to this story.