When you have a good farm, giving you lot of resources, you might want to have this for your own. Just as others. A farm can in fact be a battleground it selv, between you and your opponent.
A normal way to react when someone is breaking your sieges / killing your reinforcements and replacing their own, to keep the control, would be to attack the farm / liberate it, to get it back in your control.
This move, will certanly work. In the moment. But what you in fact do is to TELL your opponent that you are there, and you also give him a good chance to get an overwiew of what you have set into the farm to protect the resources. If you havent sent a LOT of silent ones to avoid the farm to get spied, your opponent easily can get an overwiew, and there after tune in an attack that leaves just a few of your defenders, for then to set a full raid to get most resources possible. This is NOT what you want him to do, right?
If you have a farm that is reinforced by an opponent, then simply send much more reinforces tp the same farm.. remember that your opponent HAS his forces there allready. He will NOT get a message about that you have reinforced the same farm as he has.
The chanses are small for that he will spy a farm where he havent gotten a message of, that someone has raided with his troops in.
This move will give your opponent quite a surprice. Make sure that your reinforce has a power that will ruin his offence and give him a cost of resources to build up again, that can be compared to the ammount of resources that is in the castle!
This move will probably gain that you will have tha farm alone. Or at least, there will be a big chance for it. After the first attack, then set a good amount of silent ones in, so he will not be able to spy the castle with your troops in. Do NOT give him the overwiew. In many cases, this will keep your opponent away from your farms in the future