This was the most impressive site I visited in Bali. It is a 11th-century temple complex, a series of rock-cut shrines and images, carved into a cliff face, built for a king and his wives. It is a little far from Ubud town for an easy walk, so public transport, a tour, a bus, a taxi—up to you, as we say in Asia.
You start at a long walk down, on stone cut paths, lined with the usual tourist shops. Eventually, you will come to a T junction, turn left, and there are the first set of rock excavations, large openings cut into the rock. This has been referred to as a city, and it does indeed look like a very small city. Return back the way you came, cross over a small bridge, and you will have reached the shrines. These are five small structures, cut into the rock. There is holy water here as well. From here, go back a little, and you will see a small city, with building like structures also cut into the rock.