[Solved]How to destroy a turret in Tower Defense game

Hello guys, Im in trouble with the code used to destroy turret in my tower defense (TD) game. In normal TD game, we can only place turrets to shoot enemy, and upgrade them. These things I solved already, now I want enemy be able to shoot my turrets and destroy turrets, but I can't find the solution for my code. This is my code for placing turret and user interface:

 //UI vars
 var placementPlanesRoot : Transform;
 var hoverMat : Material;
 var placementLayerMask : LayerMask;
 private var originalMat : Material;
 private var lastHitObj : GameObject;
 var allStructures : GameObject[];
 var allStructureTextures: Texture[];
 var structureIndex : int;
 static var soundEnable : boolean = true;
 
 //turrets cost
 var turretCost : int[];
 
 //GUISkin
 var style1: GUIStyle;
 var style2: GUIStyle;//blue, use when enough cash
 var style3: GUIStyle;//red, use when out of cash
 
 private var topPanelRect: Rect = new Rect(-3, -3, Screen.width+6, 28);
 private var bottomPanelRect: Rect;
 private var buildListRect: Rect;
 private var towerUIRect: Rect;
 
 //upgrade vars
 private var chosenPlane : PlacementPlane;    
 private var structureToUpgrade : TurretBase; 
 private var upgradeStructure : GameObject;  
 private var upgradeCost : int;
 var totalUpgradeBoxOpen : boolean = false; 
 var canTotalUpgrade : boolean = false; 
 var sellBoxOpen : boolean = false; 
 var canSell : boolean = false;
 var upgradeTextures : Texture[];
 
 var levelChange: LevelChange;
 
 function Start () {
     levelChange = GameObject.FindWithTag("LevelChange").GetComponent(LevelChange);
     //Calculate rect for buildList
     var width: int = 50;
     var height: int = 50;
             
     var x: int = 0;
     var y: int = Screen.height-height-6-bottomPanelRect.height;
     var menuLength: int = (allStructures.Length)*(width+15);
             
     buildListRect= Rect(x, y, menuLength+3, height+10);
 
     structureIndex = 0;
 }
 
 function Update () {    
     var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
     var hit : RaycastHit;
     if(Physics.Raycast(ray,hit,1000, placementLayerMask    )){
         if (lastHitObj) {
             lastHitObj.renderer.material = originalMat;
         }
         lastHitObj = hit.collider.gameObject;
         originalMat = lastHitObj.renderer.material;
         lastHitObj.renderer.material = hoverMat;
     }
     else {
         if(lastHitObj) {
             lastHitObj.renderer.material = originalMat;
             lastHitObj = null;
         }
     }
 
     if (Input.GetMouseButtonDown(0) && lastHitObj && !totalUpgradeBoxOpen && !sellBoxOpen)
     {
         chosenPlane = lastHitObj.GetComponent(PlacementPlane);        
         if (chosenPlane.isOpen && turretCost[structureIndex]<= LevelChange.cashCount) {
             var newStructure : GameObject = Instantiate(allStructures[structureIndex],lastHitObj.transform.position,Quaternion.identity);                
             chosenPlane.buildStructure = newStructure;
             chosenPlane.isOpen = false;
             LevelChange.cashCount-= turretCost[structureIndex];
         }
         else if (chosenPlane.buildStructure != null) { 
             //ShowTotalUpgradeBox();
             canTotalUpgrade = true;
             canSell = false;
         }
     }    
 }   
 
 function ShowTotalUpgradeBox() {
     var upOption : int;
     var sellOption : int;
     structureToUpgrade = chosenPlane.buildStructure.GetComponent(TurretBase);
     upgradeStructure = structureToUpgrade.Upgrade;    
     var x:int = Screen.width/2; var y:int = Screen.height/2;
     
     if (upgradeStructure != null){
         totalUpgradeBoxOpen = true; sellBoxOpen = false;
         upgradeCost = structureToUpgrade.theUpgradeCost; 
         for (j = 0;j<3;j++) {
             var upgradeContent : GUIContent = GUIContent(upgradeTextures[j], j.ToString());
             if (GUI.Button(new Rect (x-64,y,64,64), upgradeContent)){
                 upOption = j;
                 if (upOption == 0) {Up();}
                 if (upOption == 1) {Sell();}
                 if (upOption == 2) {Cancel();}
             }
             else x+=64;
         }        
     }
     if (upgradeStructure == null) {
         totalUpgradeBoxOpen = false; sellBoxOpen = true;
         for(k=0;k<2;k++) {
             var sellContent : GUIContent = GUIContent(upgradeTextures[k+1], k.ToString());
             if (GUI.Button(new Rect (x-64,y,64,64), sellContent)){
                 sellOption = k;
                 if (sellOption == 0) {Sell();}
                 if (sellOption == 1) {Cancel();}
             }
             else x+=64;
         }
     }
 }
 
 function Up() {
     if (LevelChange.cashCount >= upgradeCost) {
         var spawnPos = structureToUpgrade.transform.position;
         var spawnRot = structureToUpgrade.transform.rotation;
         Destroy(structureToUpgrade.gameObject);
         var newStructure : GameObject = Instantiate(upgradeStructure,spawnPos,spawnRot);
         chosenPlane.buildStructure = newStructure;
         LevelChange.cashCount-= upgradeCost;
         totalUpgradeBoxOpen = false;
         canTotalUpgrade = false;
         sellBoxOpen = false;
         canSell = false;
     }
 }

In this code, I deleted some functions which is not relevant. And the class "TurretBase" includes:

1. Upgrade: GameObject

2. theUpgradeCose : int

3. health, maxHealth : int

Class PlacementPlane includes:

1. buildStructure : GameObject

2. isOpen : boolean

My idea is that: when we click on the plane to place a turret, the variable "chosenPlane" will hold this plane, we can only place a turret to a plane which has "isOpen" is true and "buildStructure" is null.

Now when a enemy destroy a turret, I don't know how to change the variable "isOpen" and "buildStructure" of the plane where the turret was destroyed because I dont have any variables to control all the planes, I tried some ways but they didnt work. Anyone can give me some ideas about this?

Thank you very much and sorry about my bad English.