Complex sentence: The land on which the shopping centre now stands had been underwater before the 13th century, and was the original harbour for Hastings, however a series of severe storms, silting and sea erosion forced the town to move to where the current old town is.
Split into simpler sentences: The land on which the shopping centre now stands had been underwater before the 13th century, and was the original harbour for Hastings. A series of severe storms, silting and sea erosion forced the town to move to where the current old town is.