The material used to build an earthen wall 
      for a castle. This material must be harder than that used for residences 
      in order to withstand bullets and arrows, and to ensure that when a part 
      of the wall is destroyed, the rest remains stable. The earth wall is reinforced 
      with round stalks of bamboo, laid vertically and horizontally. The vertical 
      bamboo stalks are thick; their tops, sharpened like spears, project from 
      the top of the wall.  
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