The area of Yogyakarta was called Mataram in the eighth century. In Wikipedia's "Yogyakarta," we discover why there are so many ancient temples in the Yogyakarta area: "Mataram became the center of a refined and sophisticated Javanese Hindu-Buddhist culture; for about three centuries its heartland in Kedu Plain (southern slope of Mount Merapi) around Prambanan witnessed the construction of numerous candi including Borobudur and Prambanan." Candi Plaosan was built in the mid ninth century.

You can find photos of other temples in the other Indonesia pages in KoreaMosaic.