Ndalem Sasono Mulyo is a building that belongs to the Prince nDalem or a building dedicated to the Sons of the King. This building was founded during the reign of PB IV. The Ndalem Sasono Mulyo building has complete parts of Javanese buildings, consisting of 4 (four) elements commonly found in traditional Javanese houses, namely Pendopo, Pringgitan, Ndalem and Gandhok.
The Pendopo building is a joglo with 36 saka (Pillars). The pringgitan is characterized by a pyramid roof with 8 saka. The pillars in this building have a distinctive appearance because they are made in a way that is not planed, but knocked.
There is also an outbuilding along the east side which houses the courtiers (servants) who take care of the daily needs of the crown prince.
This building was previously used as a shelter for PKI members who were detained due to inadequate prison conditions. (surakarta.go.id, detik.com)