Some of the capital's long-gone landmarks are being recreated in the virtual world. Yuan Quan reports for Xinhua China Features.

Advanced technologies are bringing China's past to life - and it all started with a little girl's question for her father.

People in Beijing can now see how the city's long-demolished gates appeared almost 70 years ago by using their smartphones to overlay historical scenes on photos of the actual sites in the modern metropolis.

The technology used is augmented reality, which enables people to scan an object with their smartphone to produce a three-dimensional animation of the structure that appears to overlay the actual site.

These virtual history lessons were the brainchild of Li Yingchao, 34, an augmented reality engineer with search engine Baidu, but the inspiration came from Miduo, his 4-year-old daughter.

One day, Li and Miduo were traveling on Beijing's Subway Line Two, which follows the route of the old city wall, when the girl asked, "Why do so many stops have the word men (gate) in their names?"

The question puzzled Li. He knew that eight subway stations were named after various dismantled gates, but he had no idea why.

The Old City

Beijing, China's capital since the 13th Century, once had 20 city gates. They were built as key passageways for different social classes - including the imperial family, civil and military officials, members of the public and even criminals - to come and go from the city.

During the Ming (1368-1644) and Qing (1644-1911) dynasties, Beijing had nine large city gates. They were mostly pulled down in the 1960s, along with the old city wall, to make way for new roads and subways, and to expand the metropolis.

Records show the gates were more complicated than they appeared. A city gate, topped by a two-story tower, usually comprised a gatehouse, an archers' tower and a barbican.

The city planners only left "one and a half" city gates. The "one gate" is Zhengyangmen, on Beiijng's north-south axis at the south end of Tiananmen Square. Built in 1419, it is also known as Qianmen, or "Front Gate".

Bereft of its side walls and barbicans, Qianmen has been restored with a new face. The "half gate" is the Deshengmen archers' tower.

This was news to Li. Born in Shandong province in East China, he arrived in Beijing in 2000 to attend college. Later, he found a job in the capital, married and had a daughter. He knew little about the city's past.