The Takahashi Museum of History is a compact local museum covering the city's role in the old Kibi region, the Bitchu Province period, and the castle town that grew below Bitchu Matsuyama. It is not a blockbuster museum, but it gives useful context before or after the castle.
Visit if you want the story behind the town rather than only the famous keep. The museum is close to the station, so it is an easy add-on while waiting for a train.