Clan Castle Defense Change

So this is a really simple idea that will make the game a lot better. Give feedback please so I can make it better. Now here is what it is.

What this does is add another button to the clan castle when you tap it. It should have a barbarian icon on it. Tapping that button prevents clan troops from going out of the castle to defend. Tapping it again lets them defend. This will help by making it so donated troops for a war attack don't get destroyed before the attack, that way your clan members don't have to train another / have you request right before a war attack.

That is my simple yet great idea, again please give feedback so I can make it better, and I hope you think it is great too!