Awesome game! I can tell when somebody does their game design homework :). I appreciated being taught concepts in a straightforward and clear manner. As other comments have mentioned, the only one that felt out of place was the spider. Here's what I think of that:
1) I think the first time I encountered one, I avoided it by jumping. Thus, I didn't learn anything meaningful during my first encounter with it, I just considered it a random enemy with a linear movement. I think it would've been much better to run into a spider in a constrained space in which jumping was not allowed (like a tunnel or something). That way, it'd be clear that I can run through it. You could then follow this puzzle with a long lifting platform that had one spider patrolling it. The player would have no reason to jump, so would very likely chill there until the spider killed them. This generally would be crappy because the player would die because of an untaught mechanic, but in your game, death is not really a huge punishment since you respawn in the last platform you touched, so I think this would actually work. The alternative to this is making it obvious that spiders can pierce a shield (maybe by giving them huge fangs), but then it wouldn't be clear why they don't kill you when running. Perhaps you should also reconsider the enemy being a spider at all, and look for something that made more sense visually for this mechanic.
2) The second encounter was the puzzle with the bullets and the spider was next, and when I accidentally hit the spider and wasn't killed, I was very surprised (I even thought it was a bug). Then I figured out they only kill you when on shield, but even so, there was no clear hint/feedback as of why this was happening. There's a small animation in which the shield slowly pops, but it's not very clear why. Again, the best thing may be to remove the spider for something more intuitive.
I hope these comments make sense to you, and let me know what you think. Also, I'd love to have your feedback in my game. For the real challenge, try clearing every level by shooting only once!
I enjoyed playing your game and learned lots of game design. Thanks!