There are three layers in the temple, representing the three divisions in Buddhist cosmology, namely the world of desires, the transitional sphere, and the sphere of perfection and enlightenment. The idea of Borobudur is that walking through the corridors and layers represents the journey through life, from the world we live in to enlightenment, and students of Buddhism would walk through the monument as a way to appreciate the philosophy behind their religion. In all honesty, unless you're a Buddhist expert it's going be less a spiritual journey and more a cultural one, and the thousands of reliefs and sculptures will mean precious little, but that matters little.
For it's the scale of this whole exercise that's astounding. The reliefs alone would stretch for three kilometres if laid end to end. There are 1460 individual narrative reliefs and 1212 decorative reliefs. The first layer is covered up by stone walls – they're not sure why they were covered, but it's an indication of how Borobudur wasn't created for show, but for spiritual purposes – but the second is dominated by 504 statues of Buddha, in various hand poses. The third layer, which is circular rather than rectangular and creates a pleasing aspect to the top of the structure, contains 72 lattice dagobs (stone stupa-shaped constructions that look like large hand-bells), each containing another Buddha statue inside the lattice.
Everywhere you look are more sculptures, statues, stories and dagobs, and you can walk around the whole complex for a considerable time without repeating yourself. The sheer labour involved in building Borobudur would have been amazing, but little is known about the actual construction, just like the Pyramids and Stonehenge.
Borobudur is astounding: it's worth visiting Java for Borobudur alone. Having read about Buddhism and its history and practice, it's good to see some concrete examples of what the Buddhists achieved, architecturally speaking.
The most famous Buddhist monuments might be in Cambodia, Thailand and India, but Borobudur is still unique, and the fact that it is a stone's throw from the amazing Hindu complex at Prambanan is simply icing on the cake.
And even if you don't know anything about Buddhism, or care about the history of religion in Indonesia, it's still an amazingly atmospheric place to explore, if only because it's incredible to think that until relatively recently, the whole thing was in ruins, and someone has painstakingly worked out which parts fit where to form this monstrous and immensely moving temple. Hats off to them; it was worth the effort.