Epic Citadel app using Google's Chrome browser – specifically, the Chrome 21 release with built-in support for Stage 3D hardware acceleration can. Up until now it's been impossible to run Unreal Engine 3 on Linux because Adobe had stopped supporting its Adobe Flash Player 11, a supported Unreal Engine 3 platform and the only other viable solution for Linux users.
Google has continued to support Flash in its Chrome browser, however, bringing us to today. It's not the most ideal solution, but at least now it's a possibility. Just don't expect Epic Games to start officially supporting Linux anytime soon, even though Unreal Engine 3 ships with Linux code for developers.
"While Linux is not an officially supported Unreal Engine 3 platform, Epic provides Linux code to licensees via our own games that have utilized dedicated servers, and our licensees can ship UE3 games on Linux," Rein says. "We can confirm that users can run the UE3-powered Epic Citadel app, available at www.unrealengine.com/flash, on Linux machines, as Flash as a web solution works seamlessly across platforms. Epic continues to license Unreal Engine 3 for nine officially supported platforms, including Flash and Mac, and we have no additional plans for Linux to confirm at this time."