Complex sentence: A church at the site was present since the 9th-century, but in 1099 it was occupied by the Benedictine order, replaced in the 14th-century by the Olivetan order.
Split into simpler sentences: A church at the site was present since the 9th century. In 1099 it was occupied by the Benedictine order, which was replaced in the 14th century by the Olivetan order.