I'll agree with the general consensus, the voice acting and plot were great, as was most of the animation, but it did have some flaws. Notably, the over-use of still frames early on, the makeshift sets, and certain parts of the sound design.
The narration at the start is quite funny, and sets things up well, but things don't get going until after a quarter of the run time is gone. Then, forty seconds are used up at the end with the Knight's secret attack, leading to a slightly odd pacing, and an ending that kinda feels like an quick ending, but also feels like it only came a few seconds early. The piano jokes worked on some levels, but sometimes they kinda fell flat.
Is there any particular reason you chose that song for the end? Am I missing something, or did you just pick something random? And while I'm on the subject of sound, the noises generally worked, but I feel the knight's armor needed better sounds. It sounded like dragging metal around rather than a more clank-y sound.
On the other hand, the running joke with the princess worked even better in-film than it did on paper, and I wasn't expecting the brilliant fence-jumping jokes. The dragon fight scene looked pretty good, and the animation is smooth all throughout the short. The plot is a nice relief from the cliche, and and manages to stay fairly unpredictable and provide some nice humor.
Sure, it could have used some work in places, and the quality of the various aspects differ to a certain degree, but the sum of the parts is still good fun.