From Video Game Music Preservation Foundation Wiki
The Harp sound effect can be heard when you play the harp in the game.
The song was programmed using the MML of GW-BASIC's PLAY command and extracted from the Castle.exe program. The song can be replayed with the following BASIC command:
PLAY "MB O3 L24 C D E F# G# A# O4 C O3 A# G# F# E D C6"