Walls of castles and strongholds that had been built for hundreds of years were in cross section higher than they were wider, usually by an appreciable margin. Furthermore, they were practically vertical, which gave them the name 'curtain walls' (see the cross-section diagram). And their construction was unreinforced masonry (those who live in earthquake country understand the danger inherent in this). Heavy iron balls of a few mere inches in diameter traveling faster than the speed of sound (or near to it) had the momentum to punch holes in these walls, thereby crack the stone and mortar around the hit. All too soon, a weakened wall succumbed to gravity.