...You must level your character from 1 to 99, collect items and equipment to make him or her more powerful, and finally confront the boss of the game, Xantos, on the 99th floor of the dungeon.
Don't forget to download the manual if you want more info:
http://sourceforge.net/projects/endlessdungeons/files/The%20Endless%20Dungeons.pdf/download
There is also an FAQ on the forums here:
https://sourceforge.net/p/endlessdungeons/discussion/general/thread/6ba973f6/
This game differs from a lot of other roguelikes in that it's largely GUI- and menu based (similar to old console RPGs) and has turn-based battles.