Complex sentence: Two years after the execution of Charles I, whilst under the command of Colonel Roger Burges the Castle surrendered on 17 December 1651, with the garrison of 55 permitted to march out bearing arms and to leave the Island.
Split into simpler sentences: Two years after the execution of Charles I, while under the command of Colonel Roger Burges, the Castle surrendered on 17 December 1651. The garrison of 55 were permitted to march out bearing arms and to leave the Island.