Best way to implement this

I am coming along with my TD game. What I am trying to do is, after placing my tower objects, I want to the towers to start shooting at the enemies when they start on the path. What is the easiest way to do this. Should I use triggers? Should my whole path be a trigger area, and upon entering and within a certain proximity have my towers start shooting?

I didn't see any tutorials on working with triggers. A little insight on ways to accomplish this would be helpful.