The Medieval Wall System (MWS), constructed in the tenth–thirteenth centuries AD across parts of Mongolia, China and Russia, was one of several long walls built along ancient frontiers in Asia. Despite a growing body of literature about this network of walls and trenches, many questions still surround its construction and function. Here, the authors present results of archaeological investigations on the Mongolian Arc of the MWS, revealing new construction dates and insights into daily life. Rather than a regimented defence, the MWS, at least in parts, was a symbolic boundary that endured within the social landscape long after it was abandoned.