Complex sentence: He was kept in three different prisons over three years while awaiting his trial, which took place at the Tower of London and at which he was pardoned upon the return of Edward I from the Eighth Crusade.
Split into simpler sentences: He was kept in three different prisons over three years while awaiting trial. That took place at the Tower of London, whereupon he was pardoned upon the return of Edward I from the Eighth Crusade.