Kotabaru is Yogyakarta's old Dutch officials' residential quarter, laid out with broad streets, colonial-era houses, and civic buildings. The area includes Gereja Kotabaru, a colonial-style church, along with a monastery and the Kridosono stadium.
It is best treated as a short architectural walk rather than a single monument. Go for the quieter streets and surviving early-20th-century buildings, especially if you want a break from the royal-palace and Malioboro circuits.