Tower of God. A Korea Manga (also known as a Manwha) hosted online as a webcomic. It updates regularly and has quite a large following. It's wikipedia page is quite terrible, so I'll do the introduction below.

+Plot Summary+

There is only the Tower. A Tower that ascends upwards, with no seen limit. This is what the world revolves around.

The goal of the Chosen is to climb the tower. Through climbing the tower, they can gain fame, fortune, and power. But to climb the tower is no easy task. There are trials and obstacles, death and worse are very real possibilities. It is said, however, that at the top of the Tower, resides whatever you desire. And so they climb.

Twenty-Fifth Baam (25th night) has entered the Tower. He is not Chosen. He was not invited. But he seeks to climb to see his friend.

This is his story.
++++++

So, you can find the comic here: http://www.batoto.net/comic/_/comics/tower-of-god-r1. Reference sources are available here: http://towerofgod.wikia.com/wiki/Tower_of_God_Wiki. The Tvtropes page is also quite good as a source for additional information.

Have fun reading.