Continuing with Valve's method of orienting each episode around a particular theme or set of technologies, Episode Two focuses on expansive environments, travel, and large, nonlinear battles. Following the closing events of Episode One, it sees Gordon Freeman and the series' other major characters moving away from City 17 to the surrounding wilderness.
- Gordon Freeman
- Alyx Vance
- Eli Vance
- Isaac Kleiner
- Arne Magnusson
- Judith Mossman
- The G-Man
- Resistance members
- Xen Aliens
- Combine Forces
As was suggested in press releases, Episode Two features expansive, open-world environments in the forests outside of City 17, as opposed to the claustrophobic, urban setting of the City 17 ruins used in Episode One, particularly in its latter chapters, although it ends similarly to Episode One with a pitched battle in a fixed environment. The Muscle Car chapters are very reminiscent of the Highway 17 Chapter of Half-Life 2. Episode Two is longer than Episode One, being approximately two thirds the length of Half-Life 2.
Episode Two introduces two new enemies: the Antlion Worker and the Combine Hunter synth. As discussed in the commentary of the game, both display new, sophisticated types of A.I.: the Antlion Workers' in their standoffish approach, and the Hunters' intelligent flanking and flushing out maneuvers. With the Hunters in particular, with their resistance to conventional attacks, their deadly close combat moves and their weakness to thrown objects, the developers hoped to encourage players to interact with the game environment more closely, and try different tactics each time.
The game also introduced Antlion Grubs and Antlion Workers, the behavior of whom make the first chapters of the game different from anything yet experienced in the Half-Life games, and were also used to explain the inner workings of Antlion society. The game also gradually reveals the characteristics of the sinister Combine Advisors, and they play a much more central role to the Episode Two than in previous installments.
Although no new weapons as such are introduced, the Strider Buster, used to defend White Forest against a Combine offensive, is crucial to the last stages of the game. Valve stated before the release that they were not interested in creating new weapons, as they were more interested in exploring every dimension of the Gravity Gun, the Strider Buster being the outcome of this. This decision came in spite of the fact that the lack of new weaponry in Episode One was one of its most criticized aspects.
The game also has separated the flashlight from the HEV Suit's auxiliary power, allowing for more use of the flashlight.
Running on the so-called "Source 2007" version of the Source engine, Episode Two comes with numerous technical improvements and new features as well as those introduced in Episode One (such as Phong shading and HDR lighting). Some of the more important technical improvements seen in Episode Two are:
- Motion blur
- Dynamic lighting and shadowing 2
- Soft-Particle system
- Dual and quad-core CPU optimizations
In addition to the widely-publicized new features, there are also a number of less noticeable but nonetheless significant improvements. Lighting effects, specially for the flashlight, have been improved even further since Episode One, with any object or surface casting shadows, with the bloom effects toned down slightly to give the game a slightly less surreal look. Many character models have been improved upon once more, while the A.I. is slightly more sophisticated, most noticeably in the behavior of the new enemy - the Hunter synth. Most textures have been improved, special shaders have been applied to many characters and objects and almost everything is bump mapped.
The first two episodes of Half-Life 2 were developed concurrently by separate teams. Episode Two was originally intended to be released in two different packages: the "Black Box" – which was to ship for the PC only, and was to contain Episode Two, Portal and Team Fortress 2 – and the "Orange Box" – which was released for the PC, the Xbox 360 and the PS3, and contains all the Black Box games in addition to Half-Life 2 and Episode One. The Orange Box is now available at retail stores and on Valve's Steam content-delivery system, however, the Black Box was canceled. Instead Valve introduced a gift feature to enable those who already own Half-Life 2 and/or Episode One to give the extra copies they buy in the Orange Box away as gifts.