Any chance there might be some more updates on this game? You guys have made tons of progress in a very short amount of time, and from what I've played thus far I'm impressed with your engine. The menu system is looking pretty good as well, reminds me of SoTN. If there's one thing I'd suggest looking into, I believe your native resolution is too high for the sprites you're using. It makes it look like there's a lot of unused and empty space in some of your screenshots. Seeing as how the sprites you're using were made for games with a resolution of 256x192 (DS) and 256x240 (PS1), I'd recommend lowering it to be more in line with that. Perhaps if you change the resolution you can just upscale the graphics from there, maybe even put in some magnification filters to keep things looking nice.
Also, in addition I believe lowering the resolution would make level design easier and would make the overall look of the maps more aesthetically pleasing. Lastly, I agree with your decision to allow for solo play! The idea of a coop Castlavania is pretty cool, hell this game was sort of reminding me of Harmony of Despair when I first downloaded it. Main issue is that seeing as this is a fangame (and a Castlevania one at that, making it double niche lol), it was impossible for me to play because I don't know very many people who are interested in these types of games. Making the game exclusive to 2-player mode really limits your overall potential player-base, which IMO is already small as it is. Just my two cents.
Nice work though, looking forward to seeing more from you in the future!