Not bad, although I find it even more impressive that you are using a 3D FPS engine for a 2D platformer.
Making the level up notifications instantly pop up and modal may be something to reconsider. I'd prefer them taking place either in the background (gameplay continues as normal), or at a later "safe time" (e.g. at a checkpoint, or maybe once no monster is on screen and the player is on ground). Not sure if this falls under "UI" for you.
