Each clan gets its own battle theme and it completely changes how the stages feel. Sengoku Musou 4 (Samurai Warriors 4) ties its soundtrack to faction identity, so Takeda's "Shattered Sky" at Kawanakajima sounds nothing like Oda's "Heaven's Will" at Honnoji.
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That structural choice gives the whole campaign a sense of shifting perspective you don't usually get from Musou games. "Cherry Blossom" for Tokugawa at Osaka Castle carries this bittersweet finality that lands harder in context.
The bonus tracks pulling openings from the first three games are a nice retrospective touch. Just... the whole series in one tracklist. If you're picking one Samurai Warriors soundtrack to represent the series at its most refined, start here.