New defence: Building Guardian
In a way this defence is very similar to a Hidden Tesla, as the enemy can not see it until it pops out of the building and starts attacking.
The basic idea is that the guardian is built on top of another building (and can be changed) and it does not have any range, it just attacks whatever is attacking its building, it also doesn't have any health, it dies with the building.
It is available at th 10
Here are the stats:
Level 1: Build Cost: 6.5 mill gold DPS: 72
Level 2: Build Cost: 8 mill gold DPS: 80
I'm not sure how many you would get, maybe 1 or 2, but it would look like a dark shadowy creature and would attack with dark winds.