Ballista deal well with Doom Villages, although you'll need to chain a few, as otherwise zombies will destroy them in production. Don't delete old walls - they give you a little bit of breathing room in a worst case scenario. You'll still have quite possibly lost, but you might manage to spam enough units to deal with it. Walls can be built two deep easily, and can be staggered to be built more so. Zombies don't actually path through, they'll attack walls that are in their way, so you can build outwards from proper walls with ones to throw away. One thing I found quite useful in the final wave was building a machine turret just outside my city, surrounding it with a double wall, and then actually having a double wall with guns/towers between that and your city. But beware the power/gold/worker costs.