with more advanced lighting effects and a new version of its facial animation/expression technology. Upgrades to enemy AI allow Combine soldiers to use tactics previously unavailable to them. For example, Combine soldiers were given the ability to crouch while being fired upon so they could duck underneath the player's line of fire. The soundtrack was composed by Kelly Bailey. The music is used sparingly; it plays primarily during scenes of major plot developments or particularly important action sequences such as large battles or when encountering a new enemy. While no new locales were introduced in Episode One, extensive alterations were made to the appearance of both City 17 where the game takes place and the Citadel from the end of Half-Life 2 to reflect the changing shape of the world and remind the player that their actions have major effects on the storyline. The Citadel has degenerated from the cold, alien, and imposing fortress of the previous game into an extremely unstable state. This provides a visual cue to the player of the catastrophic damage they inflicted, and it allows for the introduction of new gameplay elements that accentuate the dangers which come with the Citadel's imminent collapse. It also serves a thematic purpose by highlighting the weakening of the Combine's dominance in City 17. Likewise, City 17 has been altered to reflect the aftermath of the resistance's open rebellion, with vast swathes of destroyed buildings, and the introduction of foes previously kept outside its confines in Half-Life 2 to emphasize the scale of the uprising. Release Episode One was sold in both retail stores and Valve's online Steam distribution system, where it was sold at a discount price. The game was also distributed by Electronic Arts as a standalone release. It was available for pre-load and pre-purchase through Steam on May 1, 2006, with Half-Life Deathmatch: Source and Half-Life 2: Deathmatch immediately available for play as part of the package. Episode One was rereleased in the Valve compilation The Orange Box for Mac, PC, Xbox 360, and PlayStation 3. About 1.4 million retail copies of Episode One had been sold by 2008. Reception Response to Episode One was generally positive, and reviewers praised the game for having more intricate, well-paced gameplay than Half-Life 2. The game's interactivity, particularly in the form of Alyx and her reactions to the player's actions and the events of the game, was also singled