Built on the next iteration of the Source engine and using Turtle Rock's next generation AI technology, Left4Dead has some serious stuff going on under the hood that might be missed by the average player who just wants to shoot things. I actually love technology like this so I poke around and read up on the various systems, not from a programmer's point of view though - I suck at code. The biggest innovation in gameplay terms is the AI Director, this disembodied figure stalks the shadowy corridors of the game environment and alters the game depending on how well (or badly) you seem to be doing.

If the AI Director thinks you are moving too slowly or getting bored, it will pep things up a little by hammering your group with a panic event. It might decide to generate a subtle musical clue to warn you, or just leave you to scream as a horde of 28-Days later zombie sprint-runners come charging at you from various spawn points that the Director thinks are amusing.

Perhaps you need a few new guns, in that case you might find a stash of them on a table in a room if you explore. The same can be said for health packs, pill bottles and the explosives. The AI Director controls normal zombie placement as well, so every replay is different in terms of the pre-placed enemies that might be lurking around waiting for you. Panic events aren't always triggered at the same time and the AI Director also has access to the sound effects, visual effects and ambient noises.

Each campaign also features a bunch of 'siege' style events (crescendo points) where you have to trigger a particular one to progress. Raising a massive metal door to get out that alerts the horde, or something similar is the order of the day here and these events are great ways for a seasoned team to show their stuff. The last mission of each campaign culminates in a big stand off against wave after wave of infected whilst waiting for a rescue vehicle to show up.

It's not just the AI in Left4Dead that's excellent, the level design is superb and the graphics are great. The model design for the infected and the survivors is spot on. The ambient special effects showcase that the Source engine has gone from strength to strength, lighting is good, the little tricks such as greying out the screen to a kind of TV-static effect when low on health gives it an edge of desperation and a cinematic quality. The animation is excellent, each of the survivors personality shines through their character model and its actions.

The infected animation is superb, combined with the physics it just shows what you can do. These zombies aren't going to die the same way each time you shoot them, some of them keep on running for a while and slowly just come to a stop when they finally collapse just before they reach your character. Or they careen off to one side and bounce off a wall, it's all great stuff. The various weapons have different forces as well, so you can imagine the shotgun is more likely to knock the infected back than the pistol.

Sound and music is top notch, the ambient sounds are great, the music is great and the starring role goes to the voice acting for the various survivors, their dialogue and interactions take the edge off the bleakness of the horror and they have a tonne of voice dialogue that is context sensitive depending on the situation at hand.