Thing is, you will need to pony up more dough if you want to enjoy Siege Mode as it will cost $3.99/€3.99/£3.19 (the equivalent of 320 Microsoft Points) in order to download and play. Basically, Siege Mode is a fourth mode that supports anywhere from a couple to half a dozen players in a game of “escort the NPC.” On one side, the BSAA squad will need to rescue a lone BSAA agent, while the other will take on the role of d-bags zombies who want to thwart the BSAA’s mission. Between a pair of rounds, players will switch between the teams in order for both sides to have a go at being human, or trying to infect one.