There are a lot of things you can say here. The title fits well to me because you write about what it is about but don't give the whole poem away. Second of all the flow is soooo smooth and beautiful. It reads very well indeed throughout. Keyhole is a great word for a few reasons as well. A poet doesn't always want his or her words to be read so its hard to see how they feel inside. And to see through that door... It was rare. The thorns are a nice metaphor for the poem because as artists we see ourselves differently. And even after we pass on our words still need to be heard. Because an artist without a voice is nothing. Great write.