Pretty interesting puzzle game. As others have mentioned the hitbox on spikes are painfully precise. A little bigger window for the error would be nice, considering that sometimes you can switch to dragon and get instantly killed because of it's huge hitbox, but i'm nitpicking. I also feel like the characters are a bit too slow. Obviously that was done for balancing purposes, so it's not that much of a problem.
Regardless, the literal approach for the Jam's theme seemed simple at first, but i understood the second layer in your realization after i finished the first level.
At first i was like "well, just get the dragon killed, easy win" but then i understood that, as you said, you change the roles, not the characters, so you will always have a villain to defeat.
And gotta say, that's pretty smart, good job! Too bad i wasn't able to defeat the princess to save the knight, heh.