Complex sentence: The open site was a marketplace established before the thirteenth century on a sloping site near the meeting point of the three hillside communities that coalesced to form Siena: the Castellare, the San Martino and the Camollia.
Split into simpler sentences: The open site was a marketplace established before the thirteenth century. The site was low sloping ground among the three hillside communities that coalesced to form Siena: the Castellare, the San Martino and the Camollia.