Just southwest of the kraton is this complex, which once served as a splendid pleasure park of palaces, pools and waterways for the sultan and his entourage. It’s said that the sultan had the Portuguese architect of this elaborate retreat executed, to keep his hidden pleasure rooms secret.
Built between 1758 and 1765, the complex was damaged first by Diponegoro’s Java War, and an earthquake in 1865 helped finish the job.
While much of what you see today lies in ruins, the bathing pools have been restored, and the tunnels and underground mosque are quite special.
(source : Lonely Planet )