Siege sees a lone AI-controlled rookie BSAA agent in trouble and in need of help. Players will split into two teams with one assuming control of enemy creatures who are trying to take down the rookie, while the other team selects playable characters from the main game and must keep him alive. Each game consists of two rounds with teams switching between the human and enemy roles.
The extra content will be available to the game sometime in March for both Xbox 360 and PS3. The content has been priced at 320MSP/$3.99/€3.99/£3.19.
Must say, fighting as the monsters seems like a lot of fun.