South Mountain’s most interesting attraction is hands-down The Mystery Castle. The whimsical castle was built by a doting Dad for his princess. Mary Lou Gully begged her father Boyce to build her a big, strong castle that wouldn’t wash away the way the sandcastles the two made together when she was growing up in Seattle did. Eventually Boyce moved to Arizona and built her a stone castle featuring 18 rooms, 13 fireplaces and tons of charming nooks and crannies.