From WiKirby, your independent source of Kirby knowledge.
Inside the Castle is a theme from Kirby 64: The Crystal Shards that plays in Pop Star - Stage 3 and Rock Star - Stage 3.
"Inside the Castle" is set primarily in the key of F major, and is essentially a combination of two music tracks from Kirby's Dream Land 3: The Last Iceberg and Friends (1). It directly uses the bassline of "The Last Iceberg" throughout the whole song, as well as the percussion and main melody of "Friends (1)", although said melody is used as a countermelody instead. It starts with only these elements (plus a unique and faint countermelody with rapid notes) for the first eight bars, and then the main melody of the song is introduced: a melancholic one, primarily featuring long notes, played by a brass instrument. All these elements play together until the main melody resolves, prompting the song to switch to a transition segment, where most instruments go quiet, but the bassline is kept intact, and the percussion becomes less frenetic, as a synth plays a slightly more hopeful melody, and a pitched percussion instrument plays a quick sequence of notes that build up in volume. The bassline is left completely alone for two bars, and the song essentially loops. The second loop is identical in terms of structure, but the main melody is instead played by the "ARR1" synth, and all instruments of the transition section but the percussion ones are different, sounding more sinister. The song then finally properly loops.
- This is one of the few tracks from Kirby 64: The Crystal Shards not present in the game's soundtrack album, Hoshi no Kirby 64 Original Soundtrack, and in particular is the only stage theme omitted other than "Catacombs". It's also the only stage theme of the game with an internal track name without the word "FIELD": its internal name is
008K4DEMOSIMOBE3, matching instead with the two songs that are remixes of the other Animal Friends themes from Kirby's Dream Land 3, Friends (2) and Friends (3). Considering all this, along with the fact that one of the Animal Friends theme went unused, and another plays in a menu, it's likely that this song wasn't originally planned to be a stage theme.