Camera stay on same Y axis

Hi!

I found a camera script in the wiki that I wanted to try out, and it worked very good. It just a camera like that I wanted, although this script and almost every other camera script I've found works the same. When you press W or S the camera flys where the mouse are pointing. I want my camera to always stay at the same level but involve a rotate function. So, what I need help with is that I want to change this following script to always stay at a specific height, not fly where the mouse are positioned, where the camera is looking.

ExFly_2.cs (FlyCam Extended)

 using UnityEngine;
 using System.Collections;
  
 public class ExFly_2 : MonoBehaviour
 {
  
     /*
     EXTENDED FLYCAM
         Desi Quintans (CowfaceGames.com), 17 August 2012.
         Based on FlyThrough.js by Slin (http://wiki.unity3d.com/index.php/FlyThrough), 17 May 2011.
  
     LICENSE
         Free as in speech, and free as in beer.
  
     FEATURES
         WASD/Arrows:    Movement
                   Q:    Climb
                   E:    Drop
                       Shift:    Move faster
                     Control:    Move slower
                         End:    Toggle cursor locking to screen (you can also press Ctrl+P to toggle play mode on and off).
     */
  
     public float cameraSensitivity = 90;
     public float climbSpeed = 4;
     public float normalMoveSpeed = 10;
     public float slowMoveFactor = 0.25f;
     public float fastMoveFactor = 3;
  
     private float rotationX = 0.0f;
     private float rotationY = 0.0f;
  
     void Start ()
     {
         Screen.lockCursor = true;
     }
  
     void Update ()
     {
         rotationX += Input.GetAxis("Mouse X") * cameraSensitivity * Time.deltaTime;
         rotationY += Input.GetAxis("Mouse Y") * cameraSensitivity * Time.deltaTime;
         rotationY = Mathf.Clamp (rotationY, -90, 90);
  
         transform.localRotation = Quaternion.AngleAxis(rotationX, Vector3.up);
         transform.localRotation *= Quaternion.AngleAxis(rotationY, Vector3.left);
  
          if (Input.GetKey (KeyCode.LeftShift) || Input.GetKey (KeyCode.RightShift))
          {
             transform.position += transform.forward * (normalMoveSpeed * fastMoveFactor) * Input.GetAxis("Vertical") * Time.deltaTime;
             transform.position += transform.right * (normalMoveSpeed * fastMoveFactor) * Input.GetAxis("Horizontal") * Time.deltaTime;
          }
          else if (Input.GetKey (KeyCode.LeftControl) || Input.GetKey (KeyCode.RightControl))
          {
             transform.position += transform.forward * (normalMoveSpeed * slowMoveFactor) * Input.GetAxis("Vertical") * Time.deltaTime;
             transform.position += transform.right * (normalMoveSpeed * slowMoveFactor) * Input.GetAxis("Horizontal") * Time.deltaTime;
          }
          else
          {
              transform.position += transform.forward * normalMoveSpeed * Input.GetAxis("Vertical") * Time.deltaTime;
             transform.position += transform.right * normalMoveSpeed * Input.GetAxis("Horizontal") * Time.deltaTime;
          }
  
  
         if (Input.GetKey (KeyCode.Q)) {transform.position += transform.up * climbSpeed * Time.deltaTime;}
         if (Input.GetKey (KeyCode.E)) {transform.position -= transform.up * climbSpeed * Time.deltaTime;}
  
         if (Input.GetKeyDown (KeyCode.End))
         {
             Screen.lockCursor = (Screen.lockCursor == true) ? true : false;
         }
     }
 }

Thank you!