ZOMBIE AI SCRIPT

Hello. This is my current Zombie AI but it just going through walls and fly. How can i fix this ?

 var target : Transform; //the enemy's target
 var moveSpeed = 3; //move speed
 var rotationSpeed = 3; //speed of turning
 var attackThreshold = 1.5; // distance within which to attack
 var chaseThreshold = 10; // distance within which to start chasing
 var giveUpThreshold = 20; // distance beyond which AI gives up
 var attackRepeatTime = 1; // delay between attacks when within range
 
 private var chasing = false;
 private var attackTime = Time.time;
 
 var myTransform : Transform; //current transform data of this enemy
 
 function Awake()
 {
     myTransform = transform; //cache transform data for easy access/preformance 
 }    
 
 function Start()
 {
      target = GameObject.FindWithTag("Player").transform; //target the player
 }
 
 function Update (){    
     // check distance to target every frame:
     var distance = (target.position - myTransform.position).magnitude;
 
     if (chasing) {    
         //rotate to look at the player
         myTransform.rotation = Quaternion.Slerp(myTransform.rotation,
         Quaternion.LookRotation(target.position - myTransform.position), rotationSpeed*Time.deltaTime);
 
         //move towards the player
         myTransform.position += myTransform.forward * moveSpeed * Time.deltaTime;
 
         // give up, if too far away from target:
         if (distance > giveUpThreshold)
             chasing = false;
 
         // attack, if close enough, and if time is OK:
         if (distance < attackThreshold && Time.time > attackRepeatTime) {
             target.SendMessage("ApplyDamage",10);
             // Attack! (call whatever attack function you like here)
         }
         if (distance < attackThreshold) {
             moveSpeed=0;
             //stop when close enough
         }
 
             attackTime = Time.time+ attackRepeatTime;
         }
 
     else {
         // not currently chasing.
 
         // start chasing if target comes close enough
         if (distance < chaseThreshold) {
             chasing = true;
         }
     }    
 }
 
 function OnTriggerEnter (other: Collider) {
     if (other.gameObject.CompareTag("Bullet")){
         chaseThreshold=100000000;
     }
 }

Please Help

//SLIMEBASS