The first settlement is defined as Paleapolis (Παλεάπολις), the Ancient Greek world for "old city", in order to distinguish it from a second settlement built during the 5th century BC, called Neapolis (Νεάπολις), "new city". The neapolis was erected towards the east and along with it, monumental walls around the whole settlement were built to prevent attacks from foreign threats. Some part of this structure can still be seen in the Cassaro district. This district was named after the walls themselves; the word Cassaro deriving from the Arab al-qsr (castle, stronghold). Along the walls there were few doors to access and exit the city, suggesting that trade even toward the inner part of the island occurred frequently. Moreover, according to some studies, it may be possible that there were some walls that divided the old city from the new one too. The colony developed around a central street (decumanus), cut perpendicularly by minor streets. This street today has become the Corso Vittorio Emanuele.