The mountain top offers a lot of excitement for the boys.  This is where they build their first, and later, subsequent, fires.  Unfortunately, the fires become something of a agme to the boys and they eventually allow them to go out in exchange for the thrill of hunting.  Eventually, there is also a lot of fear associated with the mountain top, as the boys believe they have spotted the beats there.  This preference for hunting and fear of the unknown linked with the mountain top could suggest its' symbolism of the boys' descent into savagery, which would be ironic because they have to physically cilimb to the top of it.

The platform is down below, where they have their meetings, representing civilization and order.  The fact that Golding places this below the mountain in stature could symbolize how civilization and order are of less significance on the island.