Let's bring back wall(for ground battle only)
To build walls, it's using a certain amount of house raw material
I think it should be able to hold at least 50Billions damages to make it worth or maybe 100billions to make it more interesting 
and also/maybe
Iron dome for Air Defence? to build it, need house raw and air weapon Q5. technically it's just another wall but for air battle only.
Able to hold 3 or 5 million damages for now.
In the future for sure need some adjustment due to strength growth and air rank
