At top, it lives the NPC Grak Mur , a lone pioneer.
Currently, the only way to reach it is by pressing the space bar and ascending by using the waterfalls, which sometimes can be really hard and frustrating.
EDIT: The creator of the map, lolo500 confirmed the top can be accessed by placing blocks.
At top, you will a quest, given by the living NPC, the quest is Church of Diablo.
Also, at top, temperature is really cold.