Complex sentence: William St Carileph, a much stronger bishop than his predecessor, had thus become the first head of the County Palatinate of Durham: a virtually separate state, and defensive buffer zone sandwiched between "civilised" England and the often-dangerous Northumbria-Scottish borderland.
Split into simpler sentences: William St Carileph, a much stronger bishop than his predecessor, had thus become the first head of the County Palatinate of Durham. A virtually separate state, and defensive buffer zone sandwiched between "civilised" England and the often-dangerous Northumbria-Scottish borderland.