Complex sentence: The tower of the 15th century St Mary's Church is as important a landmark to mariners as the red-and-white striped lighthouse, half a mile to the south, in warning them of the position of the treacherous sandbanks.
Split into simpler sentences: The tower of the 15th century St Mary's Church is an important landmark to mariners. The red-and-white striped lighthouse, half a mile to the south, of the church is an equaly important landmark for warning mariners of the position of the treacherous sandbanks.