How to get this code to effect all towers?

I’ve looked through this, and it may just be my lack of knowledge on this, but is there a way to make this effect everything that falls under the sprite category of “Tower”?

Right now this only effects the most recently created sprite in the Tower category, and nothing else in the Tower category. I’m trying to make a tower defense game and this is a huge pain for me, and my brain just can’t think of a solution right now.

Please help!

bump because this project is due soon