I thought the difficulty was really good; the monsters added a touch of cleverness without disrupting from the environment. I especially liked the two flyers in the vertical shaft.
For me, I found the red and yellow keys to be a bit obscure, but then I don't have a lot of patience for secret-hunting, so that's a matter of personal taste. I liked the scenery, and I liked that there was the second, darker shade of the forest, too; added some nice contrast.
If I have one issue to get stuck up on, then I it's that I felt that there could have been more than one music track; with the amount of trekking around and backtracking, it'd be nice to have more than just the one song to listen to.
But, overall, a good level, and a really good example of how a first level should be done.