A close replica of the Niko Hotel Tower (big fight scene) from the original movie, including the entrance hall, roof with helicopter and interrogation...
Action Half Life is designed to simulate the experience of being in an Action movie. Movies like Hard Boiled, Face Off, and Die Hard are the inspiration for AHL.
AHL aims for something called 'Movie Realism.' This means that you can take a few bullets and keep fighting. You can fall from a two story roof and only hurt your leg. You can bandage a wound in seconds. This doesn't mean you're invincible however, as hits to the head from large caliber weapons are instantly fatal, but a headshot even from a small pistol will put you within an inch of your life.
Of course, stunts play a large part in action movies, and likewise, play a large part in AHL. Dives, rolls, and long jumps can be executed for style and tactical advantage. There's nothing quite like diving through a window and capping the bad guy in the head before you hit the ground. This is a game of style and fun, not obsessive realism.