The domed neoclassical building behind the Rama V Memorial was originally built as a royal reception hall during the reign of Rama V, but wasn't completed until 1915, five years after his death. It's closed to visitors, but you are free to admire the mansion's graceful structure from the outside and take photos.
The hall was designed as the venue for high-profile royal functions, and on occasion it still serves this purpose. Notable among such grand ceremonies was the celebration of King Bhumibol Adulyadej’s 60th year on the throne in 2006, and more recently the coronation of King Vajiralongkorn in 2019. The first meeting of the Thai parliament was also held in the building, before being moved to a facility nearby.