I almost lost all of my lives to the boulder -> ladder jump segment but I'm happy to say I completed the demo. Boulders and lava being an instant kill is some absolute cheese though lol. It can be frustrating with the unforgiving checkpoint placement. Once I got used to the controls I had quite a bit of fun.
The slide could use some more utility. I only used it twice. Once at the beginning where it's mandatory, and the next to collect that one healing item. Perhaps steal from Megaman and have it be more useful for avoiding enemies and projectiles. It also doesn't feel very good since it kills all momentum and isn't much faster (if at all) than just walking. You also can't cancel it with a jump like Megaman does.
The game lags pretty hard if you walk during the elevator segment in the lava area. I wonder if the collision detection code is being stressed out or something. I played on web for reference. Pic below is the area I'm talking about.
Game could use some more interesting enemies. Aside from the skeleton (which I think should have slightly less health), the enemies feel kinda samey in how you deal with them.
Anyways, it's pretty cool to see some homebrew games on here. I'm looking forward to seeing how you implement magic in the next update. Nice work.