Top-Down Shooter, see through walls?

I would approach this problem like this. Have a ray casting from camera to the player. Check if something gets hit from that ray cast. If it does check if its the Bridge. If it is the bridge, then make it transparent.

So for this to work, you have to create a material and set its shader to be Transparent/Diffuse, and assign it to the bridge. And attach the following script to Bridge as well.

 using UnityEngine;
 using System.Collections;
 
 public class Bridge : MonoBehaviour 
 {
  public Color transparentColor;
  private Color m_InitialColor;
  
  void Start () 
  {
  m_InitialColor = renderer.material.color;
  }
  
  public void SetTransparent()
  {
  renderer.material.color = transparentColor;
  }
  
  public void SetToNormal()
  {
  renderer.material.color = m_InitialColor;
  }
 }

After doing that create a new GameObject and add the following script to it. Oh and drag and drop the Player transform to 'playerTrans' property on the inspector as well...

 using UnityEngine;
 using System.Collections;
 
 public class ObstructionDetector : MonoBehaviour 
 {
  public Transform playerTrans;
  private Bridge m_LastBridge;
  
  void Start () 
  {
  StartCoroutine(DetectPlayerObstructions());
  }
  
  IEnumerator DetectPlayerObstructions()
  {
  while(true)
  {
  yield return new WaitForSeconds(0.5f);
  
  Vector3 direction = (playerTrans.position - Camera.mainCamera.transform.position).normalized;
  RaycastHit rayCastHit;
  
  if(Physics.Raycast(Camera.mainCamera.transform.position, direction, out rayCastHit, Mathf.Infinity))
  {
  Bridge bridge = rayCastHit.collider.gameObject.GetComponent<Bridge>();
  if(bridge)
  {
  bridge.SetTransparent();
  m_LastBridge = bridge;
  }
  else 
  {
  if(m_LastBridge)
  {
  m_LastBridge.SetToNormal();
  m_LastBridge = null;
  }
  }
  }
  
  }
  }
 }

I've made a small video tutorial to show how this is done.

http://www.youtube.com/watch?v=dTY5_HRfICo