XI'AN -?Archaeologists said fortifications of the largest neolithic Chinese city ever discovered were excavated on Wednesday and Thursday in northwest China's Shaanxi Province.

The ruins of two square beacon towers, once part of the city wall of the 4,000-year-old Shimao Ruins in Shenmu County, have been uncovered, according to Shaanxi Provincial Institute of Archaeology.

One of the towers is 18 meters long, 16 meters wide and four meters tall, while the other is 11.7 meters long, about 10 meters wide and three meters tall, said Su Zhouyong, deputy head of the institute.

Sun said the discovery is a breakthrough and contributes greatly to archaeological research on ancient Chinese fortifications.

The Shimao Ruins were first found in 1976 in the form of a small town, and archaeological authorities only identified the ruins as part of a much larger city -- the largest of its kind from neolithic time -- last year after measuring the exact size of the ancient stone city.

The city was found to have a central area, and inner and outer structures. The walls surrounding the outer city extended over an area of 4.25 square kilometers.

Archaeologists said it was built about 4,300 years ago and was abandoned roughly 300 years later during the Xia Dynasty, the first dynasty in China to be described in ancient historical chronicles.