Attack on Titan Volume 0

Chapter 0 is the official dub title for the draft story written by Hajime Isayama which would later evolve into Attack on Titan. The draft story had a very different setting, but certain aspects of the Attack on Titan world were already in place. It was released alongside the DVD version of the first volume of the anime on July 17, 2013.

Setting
Being an early draft of the series, most things about the world were quite different from the current story. People used to build their cities inside Forest of Giant Trees (instead of Walls) because it was noted by humans that Titans didn't harm wildlife, and consequently, wouldn't harm the trees.

The Military seems to only have two divisions, although only one is ever mentioned. The Vanguard seems to would have later evolved into the Survey Corps, as they are the ones actively fighting Titans.

Titans
The Titans themselves were part of a human extermination project led by scientists and religious groups. Titans were said to be vastly superior to any living being.

Instead of being in the nape of their necks, their weak spot was located in a place inside their bodies which varied from Titan to Titan. Attacking this weak point was fatal.

Titan Shifters were also present, and were shown to have already infiltrated the military. Somewhat similarly to their current counterparts, killing them was impossible while they were in their human form, and attempting this would trigger their transformation. Only hitting their weak spots in Titan form would cause their death.

Weaponry
Instead of dual sharp blades and the 3DMG, humans were only equipped with two flaming blades that were capable of shooting energy beams. It's also shown that soldiers are able to perform super-high jumps to attack Titans.

Characters

 * Private Murakumo (むらくも二等兵) is the protagonist and a Titan Shifter. He is similar in appearance to Mikasa and Ymir, and his Titan form is the same as Eren's, but unlike Eren, he knows at the start of the story he's a Titan Shifter. He claims to have joined the Vanguard to gain the strength necessary to protect his friends. He may be based on Hajime Isayama himself.
 * Tsubaki (つばき) is a little girl who seeks to become Murakumo's apprentice. She wants to become strong so she may avenge her parents who were killed by the Titans. She looks almost identical to Eren.