I think tower/pends should count as individual, even if this involve lowering defense a little.
I've debated that.
I figured if I did so, I could leave the cost as is.
In a mono deck people would have at max 12 pillars usually, ombine with 2-4 other permanents.
It gets powerful, but take sa while to get there.
Now in a multi-element deck, particularly in some rainbows, this would get very powerful, I think however that the cost would help deter that.