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I feel like there isn't really a need to as they are really strong as they are right now as well as you already have towers you can place down and upgrade. It sounds like a unique idea, but it doesn't seem to be needed.You can deduce from the title that this suggestion is of Tower Defence.
My suggestion is to be able to upgrade the Castle Guards. We can upgrade the towers, so I think it would be nice to improve the guards. This improvement could be the speed of attack, the damage...