I did a full and throughout playthrough of the whole thing, and the experience wasn't as satisfying as I had hoped for. What I found great about the game was it's retro-style graphically and gameplay-wise.
The issue I have here is with the layout of certain levels. It seems to me that you depict a good challenge as something that you will have to overcome even if it's not apparent that a certain obstacle will be in the way or not. There were countless of times, when I get leaded by some gems into a bottomless pit, or some spikes, and not being able to see that coming is rather disappointing, since you're basically being baited into the whole thing. There were a lot of moments that required ONE certain strategy in order for you to succeed, and that strategy would usually require you to be on this one position and then head off in a certain pattern. Here, I'm talking about the level with the canonballs that you have to climb upwards in order to get a key. I wouldn't mind it as much, if you didn't also include spike balls, which really didn't make it a tougher challenge for me, as every single canonball could damage me anyway. It only forced me to wait until the right moment, and that was kind of tedious.
The other issue I have is the boss. He is humorous and funny, but the problem with him is that when you fight him in his later stages, it can become frustrating to dodge him, as his teleportation is random and isn't dependent on anything but the screen's position. This is a huge problem, because he can always teleport right in front of you when you're jumping to another platform, and you will then plummet down into a bottomless pit, and you will have to start over again. Don't even get me started on the final challenge boss. It wasn't exactly a good idea to put two of him into the same room, especially as you had a good chance of falling into a pit if you got hit by even one of them. There are challenges which are fun, difficult, and reliable, but you programmed these to be random, sometimes unavoidable, and seriously way too unreliable. I spent around an hour to get past them, and I only managed to beat them both and survive at the same time by luck, even though I managed to beat both serveral times, just to get hit by one of their canonball-barrages and then fall into a pit.
I think it's good you made it challenging, but it's honestly not good if the player is required to do an excessive amount of retries just to get past something that could well be lot's of fun to pass. It can be fixed by either fixing the terrain (not that you should do it with this game, just for future reference) and giving the player a chance of not constantly falling into holes. Or you could make the boss' teleportation pattern more predictable, something like not letting it overlap certain objects or items (and especially not the player!).