▁against ▁attacks ▁and ▁lo oting ▁by ▁rival ▁tribes . ▁The ▁migr ations ▁of ▁Tur d uli ▁and ▁C elt ici ▁proceed ing ▁from ▁the ▁South ▁of ▁the ▁I ber ian ▁Pen ins ula ▁heading ▁North ▁were ▁the ▁reason ▁for ▁the ▁improvement ▁of ▁the ▁def ensive ▁systems ▁of ▁the ▁cast ros ▁around ▁ 5 0 0 ▁BC . ▁C iv idade ▁de ▁Ter ro so ▁is ▁one ▁of ▁the ▁most ▁heavily ▁def ensive ▁cast ros , ▁given ▁that ▁the ▁ac ropol is ▁was ▁surrounded ▁by ▁three ▁rings ▁of ▁walls . ▁The ▁def ensive ▁per imeter ▁seems ▁to ▁include ▁a ▁d itch ▁of ▁about ▁ 1 ▁met re ▁( 3 ▁feet ▁ 3   in ches ) ▁of ▁depth ▁and ▁width ▁in ▁base ▁of ▁the ▁hill ▁and ▁a ▁surve ill ance ▁post ▁in ▁São ▁F éli x ▁Hill . ▁ ▁Pre histor ic ▁ne c ropol is ▁A ▁ne c ropol is , ▁with ▁seven ▁tum uli , ▁d ating ▁to ▁end ▁of ▁the ▁Ne ol ith ic , ▁early ▁ch al col ith ic ▁is ▁known ▁around ▁the ▁hill . ▁The ▁ne c ropol is ▁was ▁built ▁during ▁the ▁beginning ▁of ▁the ▁permanent ▁habit ation ▁of ▁the ▁region , ▁ 4 ▁to ▁ 6 ▁thousand ▁years ▁ago , ▁and ▁thus ▁much ▁older ▁than ▁C iv idade ▁itself . ▁ ▁In ▁Portugal , ▁these ▁bar rows ▁are ▁called ▁m amo a , ▁from ▁the ▁Latin ▁m amm ula , ▁given ▁to ▁them ▁by ▁the ▁Rom ans ▁because ▁of ▁their ▁shape , ▁similar ▁to ▁the ▁breast