Logged in just to comment on this :)
I really liked this, for it seems to be a promising start-up for a game, and you are almost there!
Actual Setup / Physics:
For the pebble is able to move really fast, you should add some accelleration for it and the mobs. This probably would need a bit more testing, for u would need some more space for longer jumps at times, but these kind of jumps would require much more skill, so you could decrease the overall movement speed a little. Adding acceleration on mobs would also give a better feeling for when they change their direction.
A few bugs I ran in:
At times, the screen started shaking, moving the picture one pixel up and down (at the third platform.) Also the character-pebble started "hopping" while not moving around. It looked kind of like an earthquake, but I m not sure if it was intended.
After running through the level 2 or 3 times and jumping into the Abyss before the final wall, i respawned directly under the first mob-spike (which killed me, then.)
U got a "Start game" Sentence to click to start the game,
And a "try again" sentence that can not be clicked. Instead, u got to click the big pebble.
--> Note: The whole concept of "having to click to respawn" was something I did not like, for I was playing the game right-handed with the arrow keys, then had to leave the keys to reach for the mouse, then reach for the keys again. Simply let the respawn happen automatically or by pressing the space-bar, so the left hand has to do something , too ^^
In the future I really would love to see several upgrades for the pebble, like a double jump, floating or even adding weapons, the small collectibles could be used as throwable ammunition, for example. Maybe, the pebble could "grow into a rock" by collecting the larger collectibles to jump a bit higher or move a bit faster or have more overall health?
U re at a good start, now u need some sounds. I would add these even before messing around with background music.
I hope u liked the feedback, for I really liked your pebble :D
to stop letting it look like s... (comment below) maybe simply change its colour to grey ;)