Ratu Boko is one of the more curious ancient sites around Yogyakarta. Debate continues over whether it was a royal palace or a religious place. The hilltop site, probably built in the eight century, has elements of both. It's a very large, sprawling site, with very few complete structures. Most of what you can see are stone walls and platforms. The platforms probably supported wooden structures, which would be typical of palace complexes.
The entire complex is surrounded by walls, many of which form terraces, but they also make it look like a formidable fortress. Within the main gate is a large open space with stone platforms that must have once held buildings. To your left as you come through the gate is a tall platform of white sandstone which has a deep well in its center. It apparently may have been a platform for cremating dead royalty.
While the large space immediately within the gate may seem empty and unpromising, if you head to the south (your right as you come through the gate), you'll find a walled compound now called the pendopo, or audience hall. Within the high wall is a raised platform, almost as high as the wall. You can see the circular indents in the stone that probably once held wooden columns in place. On the south side of the pendopo is a small pool with a shrine of the kind you can still see in Bali behind it.
On a lower level to the east of the pendopo are two large enclosed areas with pools cut into the stone slab that covers the hilltop. The pools in one of the areas are mostly round in shape, leading many to propose that it was a bathing area for the king's concubines. To me, it looks a little more like a tannery, but it may have looked more elegant in its heyday. The other area has larger rectangular pools, one of which is quite deep and thought to be sacred.
From the north side of the pendopo a trail leads up to two artificial caves thought to be meditation places.
Admission to Ratu Boko is US$10 or the equivalent in Indonesian Rupiah. However, note that there's a combined ticket available to both the Prambanan and Ratu Boko for $20. This saves you a few dollars, and there a free shuttle van that can take you from one site to the other. There's a small restaurant in the parking area of the site with a spectacular view of Prambanan, which is just over 3 kilometers away.