 On 6July 1576, a minor Mongol raid broke through a small gap in the Wall and resulted in the death of several high ranking border officials in the vicinity of Simatai, east of Gubeikou.After this incident and starting in 1577, the Ming became committed to closing all gaps along the frontier around Beijing whilst strengthening the walls.As a result, the earthen defences around Beijing were torn down and replaced by ones built with stone bricks and "sanhetu" (三合土), an early sort of concrete made of lime, clay tiles, and sand.