You can still see where the Roundhead artillery hit Richard III’s Octagonal Tower, also known as the Well Tower, at Sandal Castle. I’ve also drawn one of the forty cannon balls that were found on this slope during the excavations. The tower was already in a poor state of repair before the siege of 1645 but the bombardment reduced much of the keep to rubble.
The rectangular structure immediately to the left of the impact was a garderobe chute.
I actually helped to find one of the cannon balls Richard. I was on a school excursion to help excavate at Sandal Castle in, I think, 1968. I can’t be sure of the exact year but I know that a severe thunderstorm occurred the day we found it. The storm was so bad that when I returned home to Liversedge half the houses on our lane had smashed windows.
It would be great to get a ballistics expert in to match the cannon ball with the impact mark on the tower. It could have been your cannon ball. When you stand on the top of the motte at Sandal, you can see how vulnerable it would have been to artillery positioned on Lowe Hill in Clarence Park. I know that parts of the unlicensed castle there have been excavated, but I don’t remember any reports of Civil War finds there.