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This is because many of the things that gave the original game its appeal simply don't work in L4D2. Because of the vast differences in gameplay style over the years, the conversion of Alien Trilogy to L4D2 is, to be honest, pretty bad. The maps were also often very convoluted.
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This is back when cinematics and storylines comprised very little of a game's appeal, and where the majority of the gameplay came from learning the layouts of the maps and strategizing against specific enemy placement. As such, it follows the old DOOM-style formula of finding a key (in this case, finding a switch or battery) to open up a random door to find another switch or battery to open up another section where you must find yet more switches and batteries.
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The game itself was fairly good for its time, but it is indicative of many 90s FPS's. Alien Trilogy: Section 1 [is a custom L4D2 map based on the 1996 game Alien Trilogy for the PSX, Sega Saturn, and PC by Fox Interactive, Acclaim Entertainment, and Probe Entertainment.
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