ok it's always good to understand it at a simple perpsective, let's calm down and take it step by step.
First of all think the composer i.e. the author but it's good to use composer instead, think of what they're trying to say through this poem. My opinion would be they are trying to promote or encourage change to an individual.
Now that's a simple enough point you can keep clear in mind, so then you can think in terms of what he has done to communicate this idea, the language techniques, the door itself as being a metaphor, etc.
I think you don't have to say it so sophisticated, you say what is right, you say how you interpret this piece of text, and explain it, there's no right or wrong in english, it's how you prove it, and what evidence you have to back up your point of view on the poem.