One thing I noticed is that the archbishop seemed to fully understand what primals actually are, and exploited it to the fullest. Whereas Ysayle mistakenly thought she was reincarnated Shiva, Thordan knew that what he was summoning was the power of myth, and planned to game the system as a means of becoming God-King. He did it so well, he even took out Lahabrea for us in an ironic end.

As for why Knights of the Round, remember that the Archbishop had a praetorian guard of twelve knights styled after King Thordan's original twelve knights. This was simply the most convenient mechanism to utilize the nature of primals.