Complex sentence: In the past there were an inner sea between the hills called the Estero that eventually dried up, on which were built the "Ensanche" (Expansion or New Town), at the beginning of the 20th Century.
Split into simpler sentences: In the past there was an inner sea between the hills called the Estero that eventually dried up. On this site the "Ensanche" (Expansion or New Town) was built at the beginning of the 20th Century.