I prefere rebalancing with cost and time( to build) than gives invincibility ( except siege machines). I like walls and I'm to favor gets some realism but in multiplayer I see people's doing really strongholds with walls, very difficult for rush. I feel if we gives that characteristic the defense style becomes underpowered to overpower the good player can find the way to defeat that style but many can be frustrating. I prefer set in like AoE series. Other issue is the lag. Other issue is not so easy to build them, some times is because the turrets, if you don't have enoght resources to build a turret , the wall can be incomplete and try to close it is not realistic when that issue happens. I prefer reduces the attack to then than gives that Immunity.