Nice, good idea here. Seems to be a minor bug, sometimes you can jump and be "standing" on platforms about half way down? Might want to look into it if you're planning a future release :-)

This is really amazing! The movement was very smooth, even when I moved the floor and wall tiles. You simply got the movement down to where it feels right.

One suggestion you could change - it might make it a bit smoother (in terms of fps) if you up the speed to about half way and move in smaller steps - at the moment it feels a tad jerky to me anyway. Oh and the image of the snake going to the left doesn't seem to have a transparent background - you know you can use methods in the GreenfootImage class to just flip the image rather than having two separate ones? :-)