 Whereas European walls were mostly constructed of stone interspersed with gravel or rubble filling and bonded by limestone mortar, Chinese walls had tamped earthen cores which absorbed the energy of artillery shots.Walls were constructed using wooden frameworks which were filled with layers of earth tamped down to a highly compact state, and once that was completed the frameworks were removed for use in the next wall section.Starting from the Song dynasty these walls were improved with an outer layer of bricks or stone to prevent corrosion, and during the Ming, earthworks were interspersed with stone and rubble.