Complex sentence: The school took occupancy of west range of the abbey shortly prior to the first world war, however the abbey was initially constructed at the behest of William the Conqueror to commemorate his victory at the site over Harold Godwinson in 1066 AD.
Split into simpler sentences: The school took occupancy of the site shortly prior to the first world war. The abbey was initially constructed at the behest of William the Conqueror to commemorate his victory at the site over Harold Godwinson in 1066 AD.