Y'know after taking a fresh view at this card, I may have to rethink the stacking...I'd like to see what others think, as well.
My thoughts are that this would probably do better if it (a) stacked inversely to how it is now (as in, if you play a Pillar/Tower with two Enclaves out, the -1|-1 applies twice, but the effect doesn't reapply if a second Pillar or Tower is played); and (b) if it could drive attacks into the negative (this only makes sense to me if the former change is implemented). This would allow for a softer treatment of CC as well as keeping the casting cost on the low end of the scale.
Of course, there's always the option of having it not stack at all. I guess it could work as a backwards sort of Eclipse in that regard.
I'm a bit uncertain with this one. This is my first time working within the confines of "stack mechanics".