Hey there! Thanks for checking out!
There are a few walls with secrets~ One has an alternate boss in the Mansion~
Filters aren't really on the forefront of priorities at the moment since I'm still only about 15%-20% through development, however they'll probably make an appearance when I'm closer to being finished~ A scanline filter would be really easy to add!
As for the music- I do all of the music myself and I'm not a composer; however I feel like I need to mention that this isn't a Castlevania fangame, and as such the music doesn't try to be Castlevania music. I couldn't capture that essence if I tried because the music in CV is a masterclass in NES composition ;) I do have a number of tracks not used in the demo, and a few are uploaded here if you'd like to check them out~