If an enemy is nearby, the door will automatically close. 

private void OnTriggerEnter(Collider other)
    {
        if (other.CompareTag("Enemy"))
        {
            Debug.Log("EnemyIn");
            doorClose = true;
        }
    }

    private void OnTriggerExit(Collider other)
    {
        if (other.CompareTag("Enemy"))
        {
            Debug.Log("EnemyOut");
            doorClose = false;
        }
    }