|Shimao was protected by a thick wall (source)
The site of Shimao in Northern Shaanxi was already known but new measures indicate it covers more than 4 km² being surrounded by a thick wall, what makes it a city without almost any possible debate.
I could only find so much information on the finding but the date provided (4000 years ago) would place it between the Late Neolithic culture of Longshan
and the Early Bronze one of Erlitou
, which is typically identified with the Xia dynasty
. [But see update]
Sources in English: Mole
. In Spanish: Spanish People
Update: Va_Highlander has been looking up articles in Chinese (see comments) and it seems that the site is identified as having a long sequence from mid-Longshan to Erlitou (or Xia Dynasty).
The site is located on the Northern edge of the Loess Plateau, some 20 km away from the Yellow River.
References in Chinese language: Guancha, Smwhys and Sina.com.