Complex sentence: On the coast, west of St Agnes Beacon, on a piece of land called Tubby's Head, was an Iron Age (800 BC to 43 AD) promontory fort or quay about.
Split into simpler sentences: On the coast, west of St Agnes Beacon, is a piece of land called Tubby's Head. It was an Iron Age (800 BC to 43 AD) promontory fort or quay about.