Crunchy GBA synths giving way to sharper DS processing across two full Castlevania soundtracks in one package. Aria of Sorrow and Dawn of Sorrow back to back is ridiculous value. "Castle Corridor" from Aria has that classic handheld Castlevania feel, melody pushing through hardware limitations and sounding better for it.
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"Dracula's Fate" goes as hard as a GBA sound chip can go. On the DS side, "Pitch Black Intrusion" takes advantage of better hardware to get genuinely menacing. The jump in audio quality between the two halves is like a timeline of handheld evolution.
"Heart of Fire" sitting in there too. A classic rearranged and still hitting. Play this if you ever wondered what it sounds like when a franchise levels up its hardware mid-album.