Before examining the shrine's plan in cross section, a brief explanation of its philosophical context is called for. Borobudur is a direct reflection of Mahayana ("great vehicle") Buddhism which, during the 1st century BC, evolved from other sects called "Hinayana" ("lesser vehicle"). By contrast to the body of strict doctrines set up by Buddha himself, and spread by his disciples and the community of monks, Mahayana Buddhism took in the influences of various sects over the centuries. It is a "constituted" religion incorporating thousands of cosmogonic, philosophic, and religious variants.
Mahayana Buddhism projects a cosmos where the world is seen as an enormous sort of plateau afloat in the Absolute, in astral and cosmic voids. This plateau is organized into a system of concentric circles with, at its center, a mountain so magnificent, high, pure, and divine, as to be beyond human perception: Mount Meru.
Reverting to our cross section, we see "Mount Meru" topping three fundamental worlds - the highest being that of the subtle/spirit, followed by the solid/material/form and, finally, liquid (the world of our passions, as tumultuous as the sea): three circles, each with its individual designation.
In short, the bottom circle is strictly an account of life experiences, but almost at an animal level - the level of our needs, passions, pulsations, feelings of love and hate. Were this circle complete, this is where we would find not only scenes of war, but scenes of the erotic as well: all that man is capable of, all that man experiences. The middle circle is all that man is taught, the circle of learning, through which man seeks to distinguish himself from animals and to awaken his soul by modelling his life on Buddha and the Bodhisattvas (the class of celestial beings worshipped for being on the path to enlightenment). In the highest circle, the sphere of revelation, the body becomes superfluous, for this is where pure spirit resides. Experience, learning, and revelation: these three designations are more concrete than the more orthodox terms of, respectively, manifestation, form, and spirit.