Complex sentence: Thereafter, the site became a place of pilgrimage and a fortified monastery stood there in the 12th century, from which Isaac Comnenus negotiated his surrender to Richard the Lionheart.
Split into simpler sentences: Thereafter, the site became a place of pilgrimage. A fortified monastery stood here in the 12th century, from which Isaac Comnenus negotiated his surrender to Richard the Lionheart.