I stayed in the same harmonic progression and cues with the source but I start at a lower bpm and take my time during the intro and the first loop to get into the original bpm. Then it consists mainly of playing around with a lot of VSTs, with some additional harmonizations, a bit of counterpoint.
At the intro, I wanted to start with a cliché organ & harpsichord intro to get into a Castelvania-like atmosphere, then I played the slowed-down main melody and introduced more and more intsruments and then dissonance until it becomes unbearable, to finally get into the source's loop.
The first loop stars with a slightly increased tempo, bringing relief with cello & viola playing the melody. I aimed for a more epic / adventerous feel around here and the horns getting some parts. I used the woodwinds to add a bit of color, and to reinforce the melody especially when it was in low-mid octaves.
At the interlude I introduced the bass guitar to set the source's groove into the song, and in the second loop where I finally reach the original tempo, and put the horns in front. I also introduce rythm guitars to start to get into the power metal feel I felt appropriate for the theme.
Lead guitars get in during the interlude of the second loop and then I basically tried harmonized the melody as I would imagine a Blind Guardian song would do, up to the interlude of the third loop, then I ran out of ideas and felt the song was already long enough, and ended it by slowing it down.