Stop whatever you're doing for one moment, and listen: can you hear that noise - the one that resembles a thousand rotting feet slapping onto concrete? That's the sound of Left 4 Dead 2, bearing down on all of us like a cadaverous tidal wave. The infected hordes have already torn the balls off a half-hearted revolt by part of the L4D fan base, and soon there'll be swarming over the rest of us, pulling us apart in a frenzy of giblets and gameplay goodness.

Before that happens, however, there's still time to take a sneak peak at one of the game's scenarios. "Dark Carnival" finds our four survivors battling their way through an abandoned theme park, eventually taking a last stand inside a large rock arena. True to form, Valve has used this setting to create a vivid and atmospheric set of maps that offer a few twists on the established L4D format without compromising the core formula that made everything so enjoyable last time.

The first thing to note is that this stage takes place during the daytime. The four campaigns of the original Left 4 Dead were all set at night, and while this design choice undoubtedly helped to build a sinister atmosphere, it also meant that certain sections of the game felt a bit same-y. Dark Carnival appears to take place sometime between late afternoon and early evening, and the graphics have a sort of golden "magic hour" glow about them. Of course, you won't have much time to appreciate these beautiful surroundings as you'll be busy fighting off hoards of homicidal corpses; it's far more likely that you'll notice how much grislier the violence is this time around. There's a lot more detail to the way your rabid foes go down, thicker splashes of gore and a more variety in the way they blow to bits.

Even if you're a veteran of the first game, you may be surprised by how much claret is spilled when you set about attacking a crowd with a chainsaw or samurai sword, and since we're now fighting in daylight, the blood is a lot more noticeable. More importantly, the game's melee weapons now seem like a viable strategic option. In the last build I played you could pick items like frying pans and axes and use them to fight the Infected up close and personal - but if you took any damage, you'd immediately drop the weapon. Under those circumstances, the assorted blades and bludgeons felt a bit pointless - but now when you grab something it takes the place of your pistol. This is a much fairer, balanced trade-off: if you pick up a katana, you'll be excellent at cutting through the crowds - but if you run out of ammo on your primary weapon, you'll have no way of attacking foes from a distance.

With the rave in full swing, Barry decided to eat his glowstick

Speaking of ammunition, there are now several different kinds of ammo packs that can be used to boost the power of your primary weapon. These crates initially take up the slot used by your all-important medikit; when you're in a quiet spot you can then deploy it on the ground, creating a sort of temporary tooling-up spot for your part. One pack lets you fire exploding bullets, while another gives you incendiary rounds that make short work of crowds - and there may well be other varieties that we've yet to see. Both the flavours I encountered were a lot of fun to use, and they seem to be especially helpful when it's time to deal with one of the Special infected.

In broad terms, the level design on display in Dark Carnival is a typical mix of indoor and outdoor, shuffling between open areas and tight corridors that force the players into more claustrophobic battles. However, this general description utterly fails to do justice to the smart ideas on display. One particularly original section finds the survivors negotiating a rollercoaster track, an old school behemoth of arc, dips and wooden scaffolding. When I reached this point in the stage I made the foolish error of falling behind the rest of my team as I dithered over a choice of weapons. This kind of behaviour is never a good idea in L4D, and this occasion was no exception. A sneaky Smoker choked me and left me crying out for help on the floor, but my friends had already moved on. By this point I was far out of reach, but my buddies weren't actually that far away, geographically - they were simply walled off by bits of the track. As I slowly bled out, I could watch one chum sniping zombies from the very top of a big dipper, while another battled a Tank from behind what looked like a forest of wooden struts. By the time they noticed I was in trouble, it was too late. I died slowly, with salvation seemingly close at hand, and yet far out of reach.

Guess who?

At other points, Valve uses the scenery to spark off bits of banter between the four survivors. Ellis the redneck mechanic is the clear star in many of these conversations, getting all excited when the group approached a kids play area, and holding court with stories about his idiot friends - including one who drowned in the Tunnel of Love (naturally this tale was told just as we stepped into the ride itself, further adding to the weirdness of the situation). Naturally such touches don't have much effect on gameplay, but they still play a valuable role in contributing to horror flick atmosphere.

Indeed, many of the other additions to this sequel, particularly the new special infected types, feel like they're more evolutionary than revolutionary: they don't radically change anything, but they do give the action some much-needed variety. In addition to the rampant Charger, your new threats include the Spitter, who spits massive globs of corrosive acid, and the Jockey, who climbs aboard your head and tries to steer you into the nearest crowd of Infected. The former didn't prove too difficult to handle, on the easiest difficulty at least, but the latter was both hilarious and highly irritating - obscuring your vision while dragging you deeper into trouble.

The highlight of my playthrough, and indeed the bit that showed the most progress from the last game, arrived with the final showdown at the rock gig. Throughout the level there are posters advertising an upcoming gig for a big metal band, and when you finally arrive at the venue you find the whole place has been kitted out for a concert that never happened. Naturally it would be a shame to let all this hard prep work go to waste, and to attract the rescue chopper you need to start the entertainment systems that were meant to accompany the show.

In essence, the objective here is to survive a lengthy battle in an open, arena-like environment. So far so classic L4D, but there are loads of interesting camp points and toys that can be used to vary your strategic approach to the fight. Two high scaffolding towers rise out of the seating block, and if you climb to the top you'll find that they each hold a super-powered weapon - either a grenade launcher, or a high calibre sniper rifle. You might consider parking the whole team up here, but if you do you'll miss out on all the good stuff on stage - a pile of medikits, bottles of pills, and defibrillator that can be used to revive dead players. Even better, there's a button that fires off a massive set of fireworks; I managed to nail a tank as he was climbing up from the seating block, and the resulting inferno was highly satisfying.

We probably should have gone into battle with a concrete plan, but amid the chaos of a busy preview event, my colleagues and I ended up winging it. To no-one's surprise, the resulting rumble was a total fiasco, but we did manage to survive until the helicopter arrived. At that point my sense of honour totally deserted me, and I abandoned a fallen friend for the sake of a quick rescue. One of the other journalists managed to struggle aboard just in time, but the other two were left to die on their backs amid the frenzied zombies. We got away, but with the sour taste of shame in our mouths. For all the new bells and whistles, it's good to know core Left 4 Dead moral dilemmas remain intact.

Left 4 Dead 2 is scheduled for release on November 20 for PC and Xbox 360.