Rebuilding, 7b, Common
If your Tower was damaged
Tower +17
Else:
Wall+ 6
+17 tower from a common if you took any damage to your tower? This is far too high a value. Your opponent wouldn't be able to deal damage to your tower unless the damage is greater than 17. You'll end up with both players having three of these on hand while hoping that the other would attack some value greater than the wall, and all games would instead end with tower building or resource victories.
This can be remedied by having, like Goblin Saboteur, a minimum amount by which the tower has to have been reduced to gain the tower building effect. I'd say 12 at the minimum, arguably something higher yet.