This is quite good! There are a few flaws that make the game more frustrating than it had to be, but nothing game breaking, and the game overall is very enjoyable, and I'd love to see more of it!
The graphics are okay, they look rather bland for whatever reason, but I can't describe how exactly. The trampoline I had to jump on to get over a tree in the beginning of the game was discovered by accident, because it blend in with the tree big time. The music is great, I was just a little bit disappointed that the music doesn't change in boss fights, but that's a minor gripe.
The gameplay in this game is pretty epic, its concept is awesome, but the controls are not great. The game consists of two levels, both in a Metroid-like adventure style where you have to collect powerups and keys in order to access new places until you finally reach the boss of each stage. In your way may stand many different enemies, like skeletons that throw bones, skull spiders that glide around and this floaty thing that I swear is like an actual Metroid but more simplistic. Other than that, there are all kinds of hazards like spikes, arrow shooting bricks on the walls and all that, which are put to pretty good use in the level design, making for some action-packed and platforming intensive sections.
The set of powerups acquired in the first level is pretty nifty, going from wall jumping to crawling, to a sword (that you can use to both stab and shoot, which is cook), to a second weapon you get after defeating the boss (in a Megaman style). There are also colored keys that you have to collect in both levels in order to open the way, and that adds a simple, yet nice way to keep the whole "discovery" feel going throughout the game. The levels are nicely designed, providing some nice challenge which can be easily conquered if you make clever use of your weapons and your jump mechanics. Another nice thing about the levels is that, while backtracking is necessary sometimes, it never requires the player to run back through thousands of already visited screens, thanks to the fact that the levels are designed to more or less "loop around" towards the screen the player needs to be at, and that's pretty nice. Also, some levels provide the player with multiple options to get across certain segments, like level 2, where you can go through the first section either by jumping into an island or diving through the water, and that's also nice. I also loved the "fish power", it's hilariously useless as a weapon, but it allows the player to resist even the strongest water currents, which was a relief after being pushed around so much.
There are some problems, though, that make this game not as enjoyable as it could be. First of all, the running and jumping controls are kinda annoying, jumping is way too stiff if you're not running, and if you're running, you need to pick up enough speed for a high jump, and that eventually gets pretty annoying, and the acceleration is way too slippery as well. The wall jumping mechanics are also pretty annoying, I can't release the directional button that hugs the wall while pressing the opposite direction without the character immediately detaching from said wall, not allowing me to perform a wall jump to the other side. There is definitely a way to perform a proper wall jump in the opposite direction of the wall, but it's deep magic, I've only managed by sheer luck so far in a handful of instances, maybe give the character more adherence to the walls?
Also, while the levels are mostly well designed, some parts feel like pretty much design flaws: in the first level, for instance, there's a part where you can't move to the next screen because there's a breakable block in your way, but there's no way you can see said block because it's on the other screen! Seriously, the levels shouldn't require this kind of guessing game. The checkpoint texts sometimes obstruct the playing field, which got me killed way more often than I'd like to admit. Furthermore, the bosses feel rather lazy, they just move in the same pattern over and over again while shooting random enemies at the player.
A few final complaints: just like in Contradiction, there's no pause button, which is a pain in the butt for any action game, and the enemies respawn as soon as they go offscreen, which can get particularly annoying in certain sections. I have no idea how hard it is to code in a pause button for the engine you're using, but it'd definitely come in handy.
In spite of all that, overall, this is a pretty great game. The level design, making use of the key-and-keyhole concept, sure gave me an awesome sense of discovery and accomplishment whenever I managed to find something new, and the overall level design is action packed and quite challenging.
At the end of the second level, a text appears saying "END OF THE DEMO", indicating that you plan to continue this... I'd love to see a finished product of this, with its flaws ironed out, because this is very good!