This is a decent game but it needs to be reworked so that players won't get frustrated as they try and figure out what they need to do next. The Library of Knowledge is good for in-game lore but I found that it didn't help me in the slightest when it came to figuring out what I needed to do. I think what WILL help will be making tiles that hold items sparkle or do something to stand out so that the player doesn't have to resort to pixel-hunting to find something they need.
The combat actually isn't too bad. I was able to defeat the small demon by strafing it, and I got rid of the harpy by randomly running and spamming attack. But I will say that lining enemies up perfectly when they're constantly moving can be difficult, especially if they need to be a space or two away before you can hit them.
I haven't finished the game yet; I currently only have the Harpy Wing and the Crown (along with two keys, which I've already used), but I haven't found any other key items and I've combed the castle twice. All in all, the game just needs a bit more polish. You've got a good beginning. It just needs to be worked on a little bit more.