Duel Monsters Online is an online version of the "Yu-Gi-Oh!" Trading Card Game. The game is built using the BYOND engine, which is required to run the game.

History
Originally created by Kajika in October of 2003 as a demo proving that Yu-Gi-Oh! could be made in BYOND, the original game only included a few tables and the "Kaiba Starter Deck". This first version took about a week to make and the game was published on the HUB on the 7th of October, it instantly took off and slowly grew to have a large fanbase even in the early versions.
From there-the project started to expand, soon including the "Yugi Starter Deck" and booster pack "Legend of the Blue-Eyes White Dragon", new tables also had to be included as there weren't enough to go around.
About a month after the initial release the project was rewritten and "Metal Raiders" was included. "Magic Ruler" soon followed a few weeks later.
Around Christmas at that time "Pharaoh's Servant" was added in a single day.
Some time later "Labyrinth of Nightmare" and simple AI duelists were added. At that stage the project was considered complete, and was left as is....

However, a few months later the "Bonus"(3.0) version being created secretly, Lufia joined the staff at this time to help design it. The main feature from this version was the inclusion of Deck Masters, but many missing cards were also added. Various things were also upgraded.

At the end of 2004 version 4.0 was underway, this removed the old ranking system and replaced it with a bonus system. Several new types of dueling were also included in this version.
It was probably the biggest update DMO has had, this change shocked many players, but they grew used to the idea of unlocking various prizes.

I began to rewrite the entire system to allow me to add more to the project, this took a couple of months but it allowed many new cards to be added.

In May of 2005 the first Duelist Kingdom tournament was held. This was the first official DMO tournament and introduced a new ranking system.

Over the next few months DMO continued with small updates now and then with minor updates. In October DMO celebrated it's second birthday.

In Febuary of 2006 when 5.0 was finally released.
5.0 introduced Legacy of Darkness and online rankings, two highly requested additions. The floors were changed by creating a central hub area to walk to floors, this changed the way unlocks worked as players could now access any duel mode from the beginning.