Wow, this is a pretty fantastic game you got here! Feels very professionally-made from how beautiful it looks and how smooth it plays. It's a rather simple game, both in terms of its map design and gameplay, but it still has an addictive sense of progression to it as you slowly get kitted out and newer enemies and obstacles are introduced. The boss battles were a nice highlight as well. Nice stuff all-around!
Not to be a downer, but I do have some feedback:
*Something that struck me about the game was how silent it was: so many things that I expected to have sounds play for them didn't, and it made the world feel a bit lifeless and dull. For example, when you're in the room with those wall traps that shoot arrows, I expected to hear sounds for them arming themselves, shooting, and the arrows bouncing off things: not only would this add a lot of life to the world, sounds can also serve as warnings for obstacles and let you catch their rhythm.
*Similar to the above, I was a little disappointed at the lack of reactivity. There were a few times where I tried to do something clever like drop a box on an enemy, only for it to do nothing. I'd also like it if when you try to use a locked door you don't have the key for, it rattles or something to let you know you don't have the key. Maybe I'm asking too much, but it just made the world feel a bit less in my eyes.
*As cool as the knives are, I feel like it completely disrupts the balance of the game. Before you get knifes, you need to learn enemy patterns and formulate a route that weaves your way through the enemies, which can be quite fun and challenging. Once you get knives, though, you can just kill everything at a distance and then walk through an empty room: so boring! I hope that the game introduces something to counter-balance this later on, but I didn't get to see it in the demo.
*As nice as the game is, I think that's only within the context of playing it for free on Newgrounds for a bit: without any sort of creative mechanic, sense of challenge, or unique quality, it's just not interesting enough that I'd throw down money to play the rest of it. It's just another adventure clone to toss on the pile of millions of others.
So yeah, it's a well-constructed and nice game, but if I put it within the context of Steam and all the competition present there, it just lacks a certain je ne sais quoi to make it stand out. I still think you should be proud of making this, though, and this could just be a me thing: some people might like the simplicity and treat it as a positive which gives them relaxed vibes, I dunno.