Here are some of my thoughts and observations after watching the video.
Things I liked:
The engine is solid. All of the features such as player animations, inventory, movement, and fluidity of control are all well programmed. More often than not the thing that beaks most fangames is an engine that doesn't perform well. You guys clearly have a good foundation built and that's quite an accomplishment. Backgrounds and parallax effects are well implemented. In terms of Metroidvania engines, this is certainly one of the best ones I've played in a while.
Things that can use improvement:
As Zangetsu468 pointed out, the floating enemies seem really out of place. Aesthetically they just don't fit in with the rest of the Castlevania sprites you have, and their animations are lacking as well. For example, the floating troll only seems to have 2 frames of animation: floating and scythe swing. The beam that comes out from the scythe isn't animated very well at all, it just sort of appears and moves towards the players position at a consistent speed. Maybe adding some acceleration and onion-skinning to the beam would make it more convincing. Also, I'd highly suggest adding more frames to the enemy as well.
The floating knight enemy, same as the troll, does not aesthetically fit in with the rest of the graphics. His movement seems very stiff as he floats up and down, and he only has 1 frame of animation. You should consider using Spriter or something to animate the knight, make him breath, move his sword a bit occasionally. This would make it feel more alive, rather than being a single image that floats awkwardly towards the player. The slight up and downward movement of the knight could have some acceleration / deceleration added to give it more of a ghostly appearance rather than the sharp, abrupt up and down movement it currently has.
Since the game is 720p, be sure to make use of all of that extra screen space as in my opinion some of the rooms shown lack a lot of content for their size. You should take a look at Valdis Story: Abyssal City. It's a Metroidvania that has a larger resolution and aspect ratio like your game, and I think they did a great job constructing the levels. Almost all of the rooms have platforms to navigate, puzzle elements, and modest enemy placement to keep the player entertained.
Besides the things I mentioned above you've definitely got something good going on here.
Can you share any information about what kind of story this game is going to have? It looks like it takes place during the same time period as Order of Ecclesia which is really cool.