While the details of rituals performed here are not perfectly known, we can say that pilgrims were expected to progress through each level of Borobudur as a meditative progress that tracked the actual development of highest Buddhist consciousness.
Apparently this temple had a ritual function in relation to two other famous Mahayana temples in the same area.
Those two other temples are aligned in a straight line with Borobodur, but we are unsure about the exact details of their ritual connection. Most likely, the king and brahmins went from the first temple and then on to the others, coming finally to Borobodur at the peak of the hill.
Borobudur itself is a kind of man-made mountain.