I didn't really have to go looking for the hidden rooms since they were pretty easy to find, and apart from the 3 enemies and what, like 4 spike traps, there wasn't much to do besides walking. The Mario sprite was fun, eventhough it didn't serve any purpose (unless you'd count killing you) and the text balloons were still empty (kinda sloppy..), as for the rest, it's all been done before, the dark maze, the way you hid the rooms, the waterfall. I mean, sure, it's not like you're the only one who has done these, but I'd expect a bit more creativity in a level of 15 only screens, and especially more of a challenge.. The Medium/Challenge/Hard tag is very inaccurate, since it's more of a [Very Small/Environmental/Maze/Easy] level, in my opinion.
Make a larger level, add more stuff, PLEASE give a run powerup if you're not set on making a lot of traps based on Juni not having that powerup and truly finish it (the pointless secret room and the empty textballoons).
The first 2 screens were pretty fun to play, eventhough they were not much to look at, and after that the environment got increasingly better but also increasingly boring to 'play' (having to use 1 key per screen is boring).
Hope it helps.