Complex sentence: Castle High School was the beneficiary of a remarkably historic site: within the school's grounds are the Celtic and Anglo-Norman remains of an original Dun, or fort, flanked by a prehistoric souterrain.
Split into simpler sentences: Castle High School was the beneficiary of a remarkably historic site. Within the school's grounds are the Celtic and Anglo-Norman remains of an original Dun, or fort, flanked by a prehistoric souterrain.