Complex sentence: The end wall of the garden has holes in it made by cannon fire in the English civil war when Prince Rupert was besieging the town, which was under the control of Cromwell's troops, the Roundheads.
Split into simpler sentences: The end wall of the garden has holes in it. These were made made by people defending the town (using the holes for cannon fire) in the English civil war when Prince Rupert was besieging the town, which was under the control of Cromwell's troops, the Roundheads.