'We don't go to Ravenholm': the story behind Half-Life 2's most iconic level

At the start of Valve's 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, Freeman's former boss at the scientific "research centre" Black Mesa, looks down from giant video screens, espousing the virtues of humankind's 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. It's 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 Kleiner's pet, a debeaked face-eating alien called Lamarr, runs amok in his laboratory.