Borobudur is amazing and was the highlight of my trip. It's one of the best ancient monuments I've been to and I've been to quite a few as I have the odd compunction to travel half way around the world to see them. Borobudur is the Buddhist cosmos rendered into stone. As one ascends the levels, the visitor travels to the higher realms of cosmos. The lowest level had reliefs showing the law of cause and effect or karma but these were covered by later building at the site. Up to the next level, the life of the buddha as well as Buddhist parables have been rendered into stone reliefs.
The upper levels are the formless world with highest being being nirvana which is often translated as enlightenment. These upper levels are round with no beginning or end. They are populated with over one hundred round stupas that look like bells, each with a statue of the Buddha inside. The upper levels exude a feeling a mental peace. If you go, paying the extra money to go up for sunrise is an unforgettable experience.