Trigger not working 2D

Hello, I’m working on a simple 2D tower defense game.
I’m very new to Unity.
I have a Tower Game Object with a Circle Collider, simulating the shooting range.

Right now i only want to get a message when an Enemy enters my range.
But nothing works

This is my tower

and this is my enemy

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Tower_Collider : MonoBehaviour {

    void OnTriggerEnter(Collider other)
        Debug.Log(" TRIGGER ENTERED");

    void OnCollisionEnter(Collision col)
        Debug.Log("COLLISION DETECTED");
	// Use this for initialization
	void Start () {
	// Update is called once per frame
	void Update () {

What am I doing wrong?

You should be using

void OnTriggerEnter2D(Collider2D other)


void OnTriggerEnter(Collider other)