, ▁with ▁a ▁thick ness ▁of ▁ 3 0 ▁metres ▁at ▁the ▁bottom ▁and ▁ 5 ▁meters ▁at ▁top . ▁The ▁walls ▁were ▁sur mount ed ▁by ▁massive ▁mer l ons , ▁with ▁open ings ▁for ▁the ▁arch ers ▁and ▁the ▁long - distance ▁can n ons . ▁All ▁around ▁the ▁for tr ess ▁was ▁a ▁d itch ▁( ne ver ▁filled ▁with ▁water ) ▁ 2 3 ▁meters ▁wide ▁and ▁ 1 4 ▁meters ▁deep , ▁aim ed ▁at ▁def ending ▁the ▁found ations ▁from ▁the ▁enemy ' s ▁art illery . ▁ ▁The ▁sl anted ▁walls ▁would ▁reject ▁enemy ▁fire ▁to ▁the ▁sides ; ▁each ▁bast ion ▁consisted ▁of ▁two ▁separate ▁and ▁completely ▁self - su fficient ▁environments ▁- ▁called ▁" case ▁mat te " ▁- ▁almost ▁independent ▁g arr isons ▁on ▁their ▁own . ▁Also ▁the ▁aqu ed uct ▁to ▁the ▁city ▁was ▁devi ated ▁so ▁as ▁to ▁supply ▁the ▁for tr ess ▁first ▁of ▁all , ▁and ▁in ▁case ▁of ▁re bell ion ▁block ▁the ▁water ▁supply . ▁▁ ▁Moreover , ▁Es criv à ▁planned ▁a ▁special ▁anti - mine ▁cor rid or , ▁a ▁kind ▁of ▁empty ▁space ▁between ▁the ▁outer ▁and ▁inner ▁walls ▁which ▁could ▁be ▁walked ▁only ▁by ▁one ▁man ▁at ▁a ▁time ▁( and ▁which ▁can ▁be ▁visited ▁today ), ▁aim ing ▁at ▁def ending ▁the ▁castle ▁in ▁case ▁of ▁explos ion ▁in ▁case ▁enemy ▁soldiers ▁exc av ated ▁t unn els ▁to ▁leave ▁mines ▁at ▁the ▁found ations . ▁A ▁whole ▁hill ▁was ▁le