Sega has announced that it has acquired Three Rings Design, a San Francisco-based indie studio best know for free-to-play role playing hits like Puzzle Pirates and Spiral Knights.

According to Sega, the move is part of an “aggressive expansion into the digital space,” which also includes previously acquisitions of developers like Sports Interactive and The Creative Assembly. Sega and Three Rings have worked together in the past, as Sega served as publisher on Spiral Knights.

“Three Rings has always been a studio that prides itself on the depth and creativity of our games, we are very fortunate to have found a company like Sega that both respects our identity and shares our passion for developing engaging and quality games,” Threee Rings CEO Daniel James said. “We are honored to be joining such a legendary team and look forward to collaborating on amazing ideas together.”

Going forward the studio will continue to work on Spiral Knights, as well as on an upcoming massively multiplayer online game based on the Doctor Who series, in conjunction with the BBC.

“We are proud to welcome them as the newest member of the Sega family,” Sega’s Haruki Satomi said of the announcement, “and look forward to seeing what great things we can create together over the coming years.”