i used the artillery for a number of reasons. a couple are: there wasn't the disadvantage to the center like there is from the rings, and i could get away with leaving the disc a bit smaller if i needed to without hurting me too much unlike if i left an all or nothing from the ring at minimal defenders.

i wouldn't take retention off. if you want to add a +1 defensive dice to the ring attack for the last couple rings that would be ok, but not all of them. and not more than you have had already.