While I came to Half-Life late, Valve's 1998 masterpiece was the game that cemented my enthusiasm for games generally, and first-person shooters specifically. Replaying it almost 20 years on from its release, one aspect shines more than any other: the game's unique environments and crucial sense of place.
During the early history of the FPS, developers did not tend to prioritise settings. In classics of the form like Doom (1993), Hexen (1994) and Duke Nukem 3D (1996), settings and locations were only nominal, with little to no coherence between levels or episodes. Things began to change with id Software's Quake II (1997), in which the player progressed through numerous areas of an enemy planet, Stroggos. It was Valve and Half-Life, however, which gave the FPS its first - and perhaps best - coherent sense of place.