Mirror Wall, Common, 2b2g
Illusion.
Wall +8,
If your opponent damaged your tower last turn attack 6.
This needs to cost more. As it stands, 2 bricks 2 gems is a great price for 8 wall without condition from a Common. Having conditional attack as well as the Illusion keyword (and being amazingly cheap as an Illusion keyword trigger) sends this beyond balanced.
I do like the effect, I will say. Reacting to tower damage by building wall and dealing a normal attack is interesting in that these are somewhat useless against a tower attacking deck but excellent if your tower is getting hit because your wall was been destroyed.