Soldiers in the Three Kingdoms period (220-280) are said to build lanmaqiang, retaining walls on one side of post roads, in Guangyuan, Southwest China's Sichuan province to ensure the safety of messengers during battle.

The retaining wall stands on a side of the Jianmen Shudao, a man-made ancient post road in Guangyuan, Southwest China's Sichuan province. [Photo/cbgc.scol.com.cn]

The retaining wall in Guangyuan's Jiange county is the most well-preserved part of Jianmen Shudao, a man-made post road dating back more than 3,000 years.

The walls are mostly built at steep bends, generally about 1 meter high and 0.8 meters wide. The length depends on the situation of the dangerous road.

The buildings are firm; they are made of pebbles or big stones plus dirt or lime. The road is paved with stones with clear anti-skid marks to hinder accidents.

There are nearly 240 ancient cypress trees on both sides of a 5 km- long Jiange Shudao, which protect the roadbed for thousands of years despite wind and rain.