How was the Laolongtou built?

Lynn, legend has it that the wall was based on thousands of up side down iron woks. But the truth is that the footing stone was made of many pieces of dressed stones. Tenons and mortises were made on the stones and then a very special binder made of alum, rosin and ferrous powder was used to connect these stones. The gaps between the stones were filled with lime slurry.

After the footing stone was constructed, then the walls were built on it.