 Towards the end of the Ming Great Wall building period, skilled artisans became a prominent group of wall builders as well.During the Ming period, soldiers were in shortage due to low productivity on the military colonies, called "weisuo" (衛所) .The northern frontier, the most heavily guarded border of Ming China, was kept at 40% strength, which was equivalent to 300,000 men across a 2,000 mile border.