There's a few things you missed, but good job overall. Definitely experiment with other synthesizing programs on this song, and you should do much better now that synthesizing programs have advanced.

As to the song itself, I felt your main instrument was overpowering and abrasive. Note-wise, throughout the song it was an entire octave lower than it should've been. It also ended abruptly, as opposed to fluidly going to the next note. There was also supposed to be an echo in the main instrument. There wasn't much else that could be improved in another rendition.

Maybe take a look at mine to see what I'm talking about? Mine's a considerable amount more staccato than the original, but so far the ratings suggest it's the best cover on Newgrounds. I say that with as much humility as I can possibly convey; I didn't get as good as I am overnight, more like over four years of practice.

Not bad.

Bloodlines is such a great Castlevania game, but that's just an understatement.
The Old Sanctuary is the best song out all of the other songs from Castlevania: Bloodlines.