The multiplayer modes of Assassin's Creed III will be developed at the France-based studio, Ubisoft Annecy. The same studio developed the multiplayer portions of both Brotherhood and Revelations. VG247 also notes that the studio also worked on the multiplayer part of Splinter Cell: Chaos Theory.
Tim Browne, who goes by @n1tch, revealed himself as the "Lead Game Designer on Assassin's Creed Multiplayer @Ubisoft – Annecy" in his Twitter profile.
Officially, Ubisoft Montreal has been the only studio announced to be working on Assassin's Creed III, but this revelation should come as no surprise to anyone who's followed the development of the long-running series.
Assassin's Creed III is set to offer both competitive and cooperative multiplayer modes, and will be released on the PC, Xbox 360 and PlayStation 3 later this year.
Source: VG247
