Stronghold Crusader 2 is Steam Exclusive

The main motivation behind Firefly's decision is keeping development of the game on time
Valve's Steam platform will be required to install and play Stronghold Crusader 2 at launch, Firefly Studios announced. The company also revealed that boxed copies of Stronghold Crusader 2 will be available at retail in a number of different regions, although they will require Steam to install and play.
Here is a list of Steam features intended for inclusion in the game either at launch or through free post-release updates: achievements, voice chat, cloud saving, Workshop support, game invites, gameplay stats and Trading Cards.