The first siege was in 1643. The Royalists were on the inside, the Parliamentarians were on the outside. A Royalist sniper on the central tower, "Dumb Dyott"- so called because he was a deaf mute- managed to bring down Lord Brooke, the Parliamentary commander, with a shot to the head. Given the distance and the unreliability of the 17th century musket, that was fancy shooting.
In spite of losing their General, the Parliamentarians forced a surrender a few days later. Once inside they treated the building with- ahem- disrespect. They stabled horses in the nave, used the choir as a toilet, dug up the floor in search of buried treasure, smashed tombs and hunted cats round the interior with a pack of dogs.
On one occasion they brought a calf into the building, dressed it up in fine linen and baptised it at the font.
Prince Rupert of the Rhine, Charles I's dashing German nephew, got the building back. He did it by draining the moat, digging tunnels under the defences and blowing them up with gunpowder. This was probably the first time mining was used in British siege warfare.
During the final siege in 1646, Parliamentary artillery succeeded in toppling the central spire.
There's no medieval glass left in the building and not much medieval stonework either. Not surprising really. Several of the few, surving, medieval, carved heads carry the slash marks made by soldiers who used them as grindstones for sharpening their swords.