Me standing on top of Rochester Castle in Rochester. One of the four towers had a round outside (the others were square), and I think it's this one. Sappers dug under it during a siege (when it was square too), and it fell down. So they rebuilt it round so it would be less vulnerable to that same sort of attack. Apparently they hoped that future siege diggers would leave the other remaining square towers alone out of some sort of sense of fair play. Megan says it was rebuilt round to better repel rocks shot from catapults, we can't agree, but she's probably right.