Alright, I have finished playing the game to completion. For the purposes of a proper review, I did not use the walkthrough that you provided in the comments section. I wanted to play it as a fresh player, knowing nothing, and see how well I faired. As I mentioned, I did complete the game, so it IS possible to beat the game without any hints. However, there are a few game design choices that I would like to bring up in a minute, but let's start with the positives first.
To start, I really enjoyed playing this. I'm a big fan of the Metroidvania style of games, and so I was pretty excited to test this one out. You did a phenomenal job on capturing the Gameboy spirit, and not just because it's actually a Gameboy game. The sounds, music, and graphics really hit that spot, and it really brought me back to my childhood days. I'm also impressed with how much you got done in such a short timeframe. It's a full game. That's amazing!
Now, there are a couple of critique points that I would like to bring up. The first is quite minor, and you can take it or leave it as it is more subjective in nature. I found the button choice for jumping and attacking to feel a bit off. I'm just so used to the "A" button being jump and "B" being attack. Granted, all I had to do in the emulator was reconfigure the buttons, so I guess it's not really that big of a deal.
My second point, however, was a bit of a bigger issue. Although I was able to figure everything out in the end, I had a really difficult time doing so. I'm not saying "hold my hand through it all", but there needs to be a little bit more in the sense of providing clues. A great example is how you provided the hidden clue for the last part of the game. That was perfect! It added more gameplay, while also saving me the confusion at the end, and it still managed to remain vague enough to not give everything away. However, I couldn't really find many other clues for some of the other Triangons. I really struggled with the one in the hidden wall, and the one that you're supposed to use the fireballs for I figured out through sheer luck.
Other than that, this was a very solid game that I thoroughly enjoyed. I even broke out my USB SNES controller to play it (my NES ones don't work very well). It was a blast, and I'm glad I got to try it out. Thanks for the experience! :)