Complex sentence: The in-land bastions of the fort however are overlooked by higher ground, a fact of critical importance when the fort was besieged by John Churchill, 1st Duke of Marlborough (then 1st Earl) in 1690 during the Williamite War in Ireland.
Split into simpler sentences: The in-land bastions of the fort however are overlooked by higher ground. A fact which was of critical importance when the fort was besieged by John Churchill, 1st Duke of Marlborough (then 1st Earl) in 1690 during the Williamite War in Ireland.