Big Paul: As to why he did it....

Ruthven Castle, the main castle of the Comyn clan, now known as Clan Cumming, commanded the northern end of two passes over the Mounth, the Drumochter and Minigaig passes, while the tower at Blair, built by the first John Comyn, controlled the southern end. You can imagine the look on the Earl of Atholl's face when he got back from the crusades and found a tower on his land! He eventually saw that this was a pretty smart place to have a castle (the Comyn chief wasn't dumb) and began to enlarge upon the project. So, because of the Comyns, we have a pretty castle to look at today.