I was perusing the new games queue, and decided to check this out. I spent about half an hour playing and found it pretty fun! It reminded me of old tough NES games, in a good way. I figured I'd let you know my thoughts on the game:

I dig the level design, the difficulty actually ramps up pretty nicely. One thing that's nice to do though is give the player breaks. The moving platform section over the lava was a great challenge, but it gets a little stressful because you go so long between safe zones. Check out the Game Maker's Toolkit video below for a really great look at Mario level design that discusses this.

The castle! EPIC.

Oh no! I stomped on the boss, the music stopped, and he rolled away. Then the game just kind of stopped, with Mr Squishy Box floating in the air. I'm going to go ahead and say I won and stop there.

A couple enemy placement things I really appreciated: In level 3 there's a fireball thing on a platform. Just when I was getting the timing down for running up and stomping them, you switched it up by putting one on a moving platform. It was a surprise and kind of cool challenge. Another surprise in the cave section was when a basic enemy was placed so that you would bump the cieling if you tried jumping on them. This caught me off guard, but with the close checkpoint it didn't feel unfair.

Love the explosion / screen shake when you stomp an enemy. The flat tops of the basic enemies really invite you to stomp on them.

The controls are tight. For the most part I didn't have problems making precise jumps. When I did have trouble I think it was more due to the camera than the controls.

The camera could be improved. (cameras are hard!) I think the main thing that bugs me is that it follows every tiny movement the player makes. It's important to have a small dead zone where the player can move without the camera moving. This would help with precise jumps (especially on the moving platforms over the lava section) and keeps people from getting motion sickness from the camera moving too often.

The fireball shooter enemies could use either an indication that they're about to fire, or a slower projectile. It can be quite surprising when they shoot and you don't get much time to get out of the way, which feels a bit unfair. Their spiky heads also kind of make it look like you can't jump on them.

I like the music.

Overall a really nice project, especially if you two are just learning. Congrats on releasing a game!

Here's Game Maker's Toolkit discussing some of the level design ideas I touched on above: