▁m ▁apart . ▁On ▁the ▁out ward - f acing ▁side , ▁the ▁space ▁between ▁the ▁wooden ▁be ams ▁was ▁closed ▁with ▁lim estone ▁sheets ▁from ▁nearby ▁quar ries ▁to ▁a ▁height ▁of ▁at ▁least ▁ 5 ▁to ▁ 6 ▁m . ▁The ▁ear then ▁r amp arts ▁which ▁supported ▁the ▁back ▁were ▁around ▁ 1 0 ▁to ▁ 1 1 ▁m ▁wide . ▁An ▁estimate ▁puts ▁the ▁need ▁for ▁construction ▁materials ▁at ▁more ▁than ▁ 8 , 0 0 0 ▁trees , ▁ 1 7 , 0 0 0 ▁cub ic ▁m ▁of ▁lim estone , ▁and ▁ 3 5 , 0 0 0 ▁cub ic ▁m ▁of ▁earth . ▁It ▁would ▁have ▁taken ▁at ▁ 5 0 ▁workers ▁at ▁least ▁ 7 0 ▁years ▁to ▁build ▁such ▁a ▁wall . ▁The ▁remains ▁of ▁four ▁: de : Z ang ent ore ▁( g ates ) ▁have ▁been ▁discovered ▁as ▁entries ▁though ▁the ▁walls . ▁ ▁The ▁outer ▁r amp art ▁crossed ▁the ▁pen ins ula , ▁connecting ▁the ▁cl iffs ▁above ▁the ▁Dan ube ▁near ▁the ▁later ▁Wel ten burg ▁Ab bey ▁to ▁the ▁southern ▁bank ▁of ▁the ▁Alt m ühl , ▁a ▁distance ▁of ▁ 3 . 3   km . ▁It ▁likely ▁was ▁built ▁in ▁three ▁separate ▁stages ▁and ▁at ▁some ▁places ▁reached ▁a ▁height ▁of ▁ 6 ▁m , ▁aver aging ▁around ▁ 2 ▁to ▁ 3 ▁m . ▁A ▁mo at ▁was ▁present ▁only ▁at ▁some ▁points , ▁which ▁has ▁raised ▁questions ▁about ▁its ▁def ensive ▁character