World of Warcraft Classic will be built on the 1.12 'Drums of War' patch

Blizzard said last year that making World of Warcraft Classic is a "deceptively tough" job. It expanded upon that theme today with a development update digging down into the early stages of the process, beginning with which version of the game it wanted to build Classic on. 

"As many of you have noted, the classic period was two years long and full of changes. Core features like Battlegrounds were introduced in patches after WoW’s original launch, and class design similarly changed over time," Blizzard wrote. "After careful consideration, we decided on Patch 1.12: Drums of War as our foundation, because it represents the most complete version of the classic experience."

Patch 1.12 was the last major patch of vanilla Warcraft, meaning World of Warcraft Classic will effectively be starting from last major milestone before The Burning Crusade, its first expansion, was launched. That makes a lot of sense, though, since 1.12 is vanilla World of Warcraft at its most refined and fleshed out.

Once that decision was made, the process of actually building the game began. A "rebuilt" version of 1.12 enabled the developers to create characters and take part in "basic questing and leveling," but of course there were crashes, hardware compatibility issues, and no support for up-to-date security and anti-cheating measures.   

For a second prototype, Blizzard came at the problem from the opposite direction by using the 1.12 game data in the modern code. And that's where it all started to come together: "After weeks of R&D, experimentation, and prototyping, we were confident we could deliver the classic WoW content and gameplay without sacrificing the literally millions of hours put in to back-end development over the past 13 years," it explained. 

"While our initial effort helped us determine the experience we wanted to provide, this second prototype really defined how we’d get there. Starting from a modern architecture—with all its security and stability changes—means the team’s efforts can be focused on pursuing an authentic classic experience. Any differences in behavior between our development builds and the patch 1.12 reference can be systematically cataloged and corrected, while still operating from a foundation that’s stable and secure." 

From there, the post dives into a more technical examination of the process, breaking down the different types of data used in the game and how the formats have evolved over the (many) years that WoW has been around. Spell data, for instance, has been completely changed, and so while the current format is more efficient than the old system and not hobbled by its limitations, it also mandates that all the old data be completely reformatted—and that's not just spells, but also items, creatures, PCs, spawning, AI, and "almost every game system" that WoW uses. 

The effort will ultimately enabled "an authentic classic experience on a platform that is much more optimized and stable," and with improvements that simply were available in the early days of WoW including up-to-date anti-cheating systems, Battle.net integration, "and similar conveniences that do not affect the core gameplay experience."