Sega and Three Rings' free-to-play online adventure Spiral Knights has attracted one million registered users worldwide since its April launch. The game was added to Valve's popular Steam service early last month, which caused a spike in registrations. "Seeing a million eager knights forging ahead into the Clockworks, building guilds and our game community, is gratifying for our entire team," said Three Rings CEO Daniel James in a statement. "As seen in content updates like today’s Ironclaw Munitions Factory and the recent Gloaming Wildwoods update, we are more determined than ever to continue expanding and adding to the Clockworks for our fans," he continued. Spiral Knights was added to Steam as part of its initial push of free-to-play offerings on June 14. Valve later followed up that initial batch with one of its own titles, changing Team Fortress 2 into a free-to-play game.