Shooting Specific GameObject via Tag

What do you mean by "certain"? Based on distance or some other targeting criteria?

I mean just a GameObject with a specific tag

Do the towers have a range? You could have a trigger on the tower and when a gameobject enters it, check its tag and start shooting if it's the appropriate one. This would probably be the most simple way, but you could do it heaps of other ways, depending on how you want the tower AI to work. eg:

 public string targetTag = "Red Team";
 
 void OnTriggerEnter(Collider other)
 {
    if(other.gameObject.tag == targetTag)
       Shoot(other.gameObject);
 }

@liamcary is right. That's the simplest way to do it.

@darkartsprodigy, you need to be more specific with the behaviour of your tower, so we can figure out which way is better for you.

You can also use distance:

 if (Vector3.Distance(target.transform.position, transform.position) <= minShootingDistance) {
     Shoot();
 }