Setting boundary for Z axis

Hi all, I have created a simple game which the player can move on it's X and Z axis and right now I am facing problems in setting up my game boundaries, I was able to set the boundaries for my X axis but not for my Z axis.

This was what I wrote for my update function in C# script

void Update () {

     //boundaries for X
     float xPos = Mathf.Clamp (transform.position.x, -5,6);
     transform.position = new Vector3 (xPos, transform.position.y, 
                                       transform.position.z);

             //boundaries for Z
     float zPos = Mathf.Clamp (transform.position.z, -14, -25);
     transform.position = new Vector3 (transform.position.z, transform.position.y,
                                      zPos);
     //movement
         moveToPosition = transform.position;

     if (Input.GetKey (KeyCode.A)) {
                     moveToPosition.x -= speed *Time.deltaTime;
             }
     if (Input.GetKey (KeyCode.D)) {
         moveToPosition.x += speed *Time.deltaTime;
     }
     if (Input.GetKey (KeyCode.W))
         {
         moveToPosition.z += speed *Time.deltaTime;
     }
     if (Input.GetKey (KeyCode.S))
     {
         moveToPosition.z -= speed *Time.deltaTime;
     }
     transform.position = moveToPosition;

 }