Complex sentence: By the 14th century Bristol, York and Norwich were England's three largest medieval towns after London, but one-third to one-half the population died in the Black Death of 1348--49.
Split into simpler sentences: By the 14th century Bristol was one of England's three largest medieval towns after London, along with York and Norwich. Between a third and half of the population were lost during the Black Death of 1348--49.