Along with hordes of fast-moving 'common infected', there were five types of 'special infected' the director could deploy to mix things up, and it did.

This is the swimming pool you fill with zombies in Dark Carnival. Scavenge mode borrows locations from the main game. An 'AI director' varied the volume and location of enemies depending on your skill, teamwork and previous experiences, and the level design kept you on the back foot with no idea where the next threat would come from, even while it simultaneously left you in no doubt where to go next. Behind the scenes though, Valve did a lot of heavy lifting. On a basic level, you chose a big gun and a pistol and fought zombies until you got to the next safe-room, and then pressed a button at the end and defended against waves of enemies until help arrived. The first game was a ballache for Valve to produce, but the results were simple for players to understand, because it had good rules and well-worked systems. You pick from one of four characters, choose a campaign and then work with one another to survive the zombie apocalypse the game unloads between the starting point and the last chopper out of town. On the off chance you didn't play the original, Left 4 Dead 2 is a first-person shooter designed for four players to play together. Because people really, really did get it. But they are symbolic of a broader change to a game that looked to some like a cash-in: whereas Left 4 Dead was about getting people to co-operate, and a nervous Valve was reluctant to complicate things in case people misunderstood, Left 4 Dead 2 is about making the most of the fact people did get it, by having some fun with the concept. When you first encounter them halfway through the opening campaign, Dead Center, they're not even the best thing in that room - the man who wants you to go across the road and get him some Cola is. The laser sights aren't the best thing in Left 4 Dead 2.

But attaching laser sights to your weapon certainly makes it look cool. It's hard to verify, even after you've shot 7500 zombies during your first day playing it. Actually, I have no idea whether attaching laser sights to your weapon in Left 4 Dead 2 makes it more accurate. 'Attaching laser sights to your weapon makes it more accurate.' Yeah right.

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