Complex sentence: There are three related types of Neolithic earthwork which are all sometimes loosely called henges, the essential characteristic of all three types is that they consist of a ring bank and ditch but with the ditch inside the bank rather than outside.
Split into simpler sentences: There are three related types of Neolithic earthwork which are all sometimes loosely called henges. The essential characteristic of all three types is that they consist of a ring bank and ditch but with the ditch inside the bank rather than outside.