Destroying a prefab on collision with a cube?

Hey, I'm relatively new to Unity3D, and I have to make a game for a piece of course work. The game I am making is a 2D shooter, where the play controls an object that can be moved left and right at the top of the screen. The objective is to protect a cube in the bottom center of the screen, while enemies attack from the left and right. I want the enemies to be destroyed when the hit the cube, but I can't get it to work! here is the code I've been trying to use, which has worked in other parts of the game:

 function OnCollisionEnter(collision : Collision) {
     if(collision.gameObject.tag == "Player")
     Destroy(gameObject);  
     }

The enemies come from a prefab called SpawnPrefab, and the cube is called Monument. I have tagged both the cube and the prefab/target as player, but it doesn't work! Thanks in advance! :)