Hey Cameron,

Hope all is well.  When you are trying to sort out the point of a parable just remember there is only one point.  To use the word allegory when speaking about the scriptures is frowned upon because the sense is that once you give spiritual meaning to everything, for that is what defines the allegory, you diminish its historicity.  Maybe. Maybe not. Remember that people stumble over these things sometimes so you’ll want to tread lightly when talking about this.  John 10, in response to your question, has more metaphor than allegory, even though the widely accepted definition for allegory is simply an extended metaphor. 

John 10 begins…Truly, truly, I say to you, he who does not enter by the door into the fold of the sheep, but climbs up some other way, he is a thief and a robber. But he who enters by the door is a shepherd of the sheep.  To him the doorkeeper opens, and the sheep hear his voice, and he calls his own sheep by name and leads them out.  When he puts forth all his own, he goes ahead of them, and the sheep follow him because they know his voice.  A stranger they simply will not follow, but will flee from him, because they do not know the voice of strangers.”  This figure of speech Jesus spoke to them, but they did not understand what those things were which He had been saying to them

The author uses a contrast to draw your attention to the main point he is making.  He is contrasting something.  This is where you may have gotten tripped up.  You will want to give meaning to the porter.  Not only is that a fruitless endeavor but it also looses sight of what a parable is doing, namely making a single point.  Allegory will want to spiritualize everything. Though that is entertaining it is also misleading.  A great question to help you through this is, “For whom will the porter open the door?”  The answer to that question will show you how insignificant the porter is in the parable.  Another great question is, “What distinguishes between the response the sheep have toward the porter and the response they have toward thieves?”  You will also want to resist eisegeting the thieves and robbers as well.  Don’t try to squeeze the Pharisees in there.  Let the author simply contrast verse one with the rest of the paragraph.  Ask the question

  1. How do the thieves enter?
  2. What is the implication?
  3. How does the shepherd enter?
  4. What determines whether or not he is the shepherd?
  5. Who else will the porter open the door to?
  6. What is being entered into?
  7. What is a sheepfold’s purpose?
  8. How does the shepherd motivate the sheep to follow him?
  9. Who are the sheep afraid of?
  10. How well do the sheep know the shepherds voice?
  11. What does that imply about the relationship?
  12. Do they follow sight or sound?
  13. What is significant about that?
  14. How well does the shepherd know the sheep?
  15. What is the shepherds intention for the sheep?
  16. What are the intentions of those who cannot use the door?
  17. How do sheep respond to the voice of the stranger?
  18. Who in this parable is a stranger?
  19. What exactly is a stranger?
  20. What is verse 2-6 describing?

So…What is the difference between the thieves and the shepherd? Therein lies your contrast. 
The next paragraph tells you that Jesus is the door.  There is an internal contrast that is important when Jesus gives a temporal reference point. 

Jesus tells them He is the door, and that he isn’t a thief.  When He says “All who came before me were thieves and robbers,” He is also letting them know that  ‘As of this point in time something has changed.’. Ask the question how did the people respond to the ‘All’ who came before Jesus if they were thieves? 

Then, when Jesus tells the parable of the good shepherd, he is drawing another internal contrast between the hirling and the good shepherd.  What is the eventual result of the sheep when the hireling is on the job? What happens to the sheep?  How does that differ from the result of the good shepherd performing  his role? 

Honestly the good shepherd is not the parable, the sheepfold is(verses 1-6) the door and the good shepherd are just additions to help them understand the point He is making. 
Finally, and this is important as well, recognize that the first 18 verse of John 10 are sandwiched in by the blind man material, which makes the parable of the sheepfold and its main point hinging on what is happening on both sides of it. Think, the people are afraid -what are they afraid of? What did the disciples ask Jesus in 9:1-12? What was their understanding about the cause of blindness?

Gosh, I hope this helps.  Let me know what you come up with.