A few suggestions:
1. An instruction on using mouse for picking up stuff is needed
2. At level 14 the solution seems to be picking up the key from ACTIVE spikes tile, wich is definitely not obvious.
3. The fact that you still have items you picked up (crates, slimes) after "death" is not obvious and needs to be mentioned in instructions
4. In level 16, it is possible to pass through left wall and game freezes (the second rile from the uppermost, if I remembered correctly) Also, it is possible to push the slime to the right of it through the wall
5. The character sometimes isn't responding to certain leys, like there's an obstruction