The best part of Uncharted 2: Siege
is, as the name suggests, Siege. On a basic level, Siege isn't much more than a variant of Survival. The key difference is the addition of up to two friends to help you fend off waves of enemies. The idea isn't anything new, but Naughty Dog has managed to find ways to leverage the already existing play mechanics and create something just a bit different.
The goal is to withstand ten waves while restricted to small areas of the map. Once all three members enter the specified area, a wave of enemies rush towards your position. Meanwhile, a timer counts down the seconds until you have to run to another zone and face down a second, more powerful wave. Again, it isn't exactly new, but the smaller areas add a frantic pacing. It's not like you can run and hide, but you also can't make a valiant Rambo-style stand. If an ally falls, you can revive them but at your own personal risk. Even if they fall behind cover, enemies come from all sides, so every save is a gamble.
What I liked most about Siege is how much importance it places on communication and strategy. I love when co-op games actually force players to communicate. A great example involves melee enemies, who will charge and grapple with players. You can always shoot the enemy off your friend, but also risk hitting them in process. The alternate is to charge with your own melee moves, placing a giant target on your back. Matches alternate between moments of cheerleading and yelling at each other for sloppy play. It's great!