At the start of Valves Half-Life 2, the seminal first-person shooter game that turns 20 this month, taciturn scientist Gordon Freeman is trapped within a dystopian cityscape. Armed soldiers patrol the streets, and innocent citizens wander around in a daze, bereft of purpose and future. Dr Wallace Breen, Freemans former boss at the scientific research centre Black Mesa, looks down from giant video screens, espousing the virtues of humankinds benefactors, an alien race known as The Combine.As Freeman stumbles through these first few levels of Half-Life 2, the player acclimatises to the horrible future laid out before them. Its hardly the most cheerful setting, but there are some friendly faces (security guard Barney, Alyx and Eli Vance) and even moments of humour, as Dr Isaac Kleiners pet, a debeaked face-eating alien called Lamarr, runs amok in his laboratory. It feels safe. It feels fun. It feels familiar. Theres even a crowbar! And then, the foreshadowing. Thats the old passage to Ravenholm, mutters Alyx Vance during Freemans chapter five tour of the Black Mesa East facility. We dont go there any more. You feel a shiver down your spine; you know you will end up going there.[Ravenholm] was a totally different environment from what the player had been in until that point, says Dario Casali, level designer and member of the informal City 17 Cabal, a group within Valve that worked on Half-Life 2s most famous level. It was an outlier of a map set that survived from a pretty early build of the game, borne from a need to give the newly introduced Gravity Gun a place to shine.Ravenholm was a totally different environment from what the player had been in until that point. Photograph: ValveThe absence of ammunition for Freemans traditional weapons is the impetus that drives Ravenholm and Half-Life 2 into horror game territory. An old mining town, previously hidden away from the Combine, Ravenholm is now a desolate place, plunged into darkness, its citizens corrupted by an intense bombardment of headcrabs (those face-eating aliens). We made use of confined spaces so that slow zombies [headcrab-afflicted people] could actually get near you, reveals Casali. And the player can no longer blast them away with a machine gun or pistol; you need to resort to the hefty Gravity Gun, picking up whatever you can find around you and flinging it at the monsters bearing down on Freeman. Pots of paint, bits of wood, even dead bodies became the players ammo.Like most of Half-Life 2, Ravenholm is a cinematic experience, taking its cue from horror movies such as Saw and 28 Days Later. When Combine forces attack Black Mesa East, Freeman escapes through the dark tunnel that leads to Ravenholm. Instantly, a sharp change of atmosphere descends like a chill upon the player: a grim set of dark buildings, wispy, almost nonexistent music, two crashed headcrab rockets, and something swinging from a barren tree. Closer inspection reveals the lower half of a corpse, pecked at by crows.Headcrab zombies appear out of nowhere, moaning their pained exhortations; but soon these are the least of Freemans worries. Designed to fit around the map, Ravenholms fast zombies climb up drainpipes and scurry across rooftops, leaving little safe haven for the adventurous scientist. Freeman must also contend with hunched creatures that hurl poisonous headcrabs.A desolate place. Photograph: EAFortunately, Freeman is not without help; soon, he encounters Father Grigori, who is responsible for Ravenholms Saw-like traps and passionately redeems his flock with a shotgun. Casali says: My take on it was that this guy had slowly lost his mind because of the headcrabs and the conversion of his congregation into zombies. Because Ravenholm was so isolated, I imagined he didnt even know about the Combine invasion and thought that the devil had come to town. Father Grigori and his flock of zombies was the perfect excuse to double down on the creepiness.Freeman follows Grigori throughout Ravenholm until a final climactic battle in (appropriately) a cemetery. I thought Ravenholm really needed a fittingly action-packed ending worthy of a horror film, says Casali, and what better place to do that than in a graveyard!While that closing encounter, with Freeman and Grigori besieged by an army of zombies and headcrabs, releases some of the tension built up while exploring the creepy streets of Ravenholm, the level still leaves a lasting impression on anyone who played it, such is the abrupt change of tone and style. The segment endured practically from the start of Half-Life 2s long development a version appeared in Valves renowned 2003 E3 demo evolving into the ammo-scarce spook-fest of the final game.As one of the outstanding games of the last 20 years, Half-Life 2 defined the future of video games with its innovative visuals and remarkable physics engine. As a part of the City 17 Cabal, Casali and his colleagues work was instrumental. The desire to outdo the original Half-Life was so strong, and we were constantly motivated by the quality of work the other teams were doing, he remembers. It was magic.