While it can be botted to auto-attack people after the shield goes up...if you are away and you get attacked, the shield means that you may have a chance to not lose all your resources. Without the shield, people will farm everyone to their safe levels. A short shield of a few hours isn't that bad.

Might I suggest the addition of a hospital to recover a certain percentage of your troops, possibly with some additional cost.

Right now, when you get attack, perhaps while sleeping, even if it's not really that financially beneficial for them, you lose all your troops. It can take quite a bit of time to recover your army. That leaves you vulnerable and means you can't really leave it safely.

I'd also suggest selling "boosts", including items that enable the shield for different lengths of time. Items that people want to buy to give them an advantage help add to the pool and make it more likely to be continually profitable for everyone.

Edit: An alternative for the shield would be to only activate like a 3 hour shield or something when the player hasn't been attacked within a certain time limit, like say 24 hours or something. That means that if they sign in soon enough after getting attacked, they can save some resources, but they go into a mode where they can be farmed after the initial attack.