What went wrong: I don't really have much to say here. I really liked the piece in question. What I do have to say is that it seems barely incomplete, like there was one final thought you wanted to get in but didn't quite manage to convey. The other thing being, your final couplet felt the tiniest bit awkward.
What went right: Your rhythm and rhyme were all but impeccable, and that was enjoayble to see (Pointless side note from the critic: I'm sick of seeing poorly rhymed poems for the sake of having a rhyme scheme). Also, I liked the source material. You're being playful and light with your work instead of trying to consistantly conjure the heaviest of emotions from the reader. This poem appeals to the lighter side of things. Good job. Also, your use of enjambment was well done and actually served to increase the reader's interaction with the poem.
How to improve: Maybe revise a few times to see if you can't make that last couplet a little less unwieldy? Also, maybe think about the completion thing... Or not, I could just be crazy.
Overall score: 9/10. Exemplary work!
Eternally devoted and wed to my darling pet, Nagoyaka Aikouka.
"Your Grace, all that you say is true. On the Trident, Rhaegar fought valiantly, Rhaegar fought honorably, Rhaegar fought nobly, and Rhaegar died.