You've discovered an ancient manuscript containing a mysterious riddle: 'I am the beginning of eternity, the end of time and space, the beginning of every end, and the end of every place. What am I?' This riddle is said to hold the key to unlocking a hidden door in an old temple. Write your thought process as you work through solving this riddle. Consider multiple interpretations and explain your reasoning. Once you've determined your answer, describe what you think might lie behind the hidden door and what its significance might be to the ancient civilization that created it.