Really loved the simplistic approach to the game design.
The pixel art was neat and the colors were cool.
You could have added in some visual aesthetics like some clouds and stuff instead of some plain old Blue back ground !
I noticed something with the block, it's mouth opens only when you press the left or right keys and immediately closes it when you try to jump.
The game is not all too difficult to name it The Impossible game, because timed jumps can get you easily to the final level! Actually anybody could make a speed run to the boss, except to have 0 clue on how to defeat him.
I got to the Boss level and dodged him for a bit and got to the other end, but nothing happened so I decided to jump on the boss like in Mario to kill it but I died instantly(Unfair).
So the next option was to get to the door with a red(electric?) barrier and try to pass through it, but every gamer's instinct is to avoid barriers and I still decided to go through which evidently killed me....
In the end I gave up and I would love to hear on how to beat the boss!!
Please don't tell me-you can't and hence the name "The impossible game".
P.s. I liked the game a lot, and I hope you add more levels to it with different traps instead of the same spikes at every level.
You could also add in some kinda scoring feature like collecting coins and such.
This game also needs a Level complete display to pop up because when you complete a level by landing a long jump and pressing the arrow keys in one direction the block tends to continue moving in that direction and I at multiple times walked into the spikes in the next level(because of this), meaning there is no break in action !
This could be overcome by either creating a level complete display with points obtained so it gives the player time to relax or you need to extend the region on obstacles away from the starting point (This happened when I completed level 3 and walked straight into the spikes in level 4 which were too close from the starting point!)
This is just my opinion.
Loved the music and the game over sound would play full till it restarted the level, it could be better if you could skip the tune if you want by pressing a key to restart immediately!
Good Luck further :)