Doom II - Hell on Earth - v1.9 id Software 1994 Source port used : ZDoom v2.4.1
** The Story The player once again takes the role of the Doomguy, who, after being stranded on Phobos and subsequently fighting his way out of Deimos and Hell itself, returns home to Earth — only to find that it too has fallen victim to the hellish invasion.
With all the major cities in the world in ruins, the remaining leaders plan to use spacecraft to transport the survivors of Earth's population. However, the starport is the only way for the ships to depart and the demons have protected it with a force field. All of humanity's remaining soldiers make a desperate assault on the starport, but eventually they are decimated and only the player remains. He continues on doggedly and selflessly, despite knowing that he will be left behind in order to save the rest of his race.
Once the Marine accomplishes this, he is free to live out the rest of his time alone on Earth while humanity hopefully continues on elsewhere. But along the way, he begins to learn how he might finally thwart the invasion once and for all... (extract from the Doom Wiki)
** Notes The ZDoom game engine is slightly different from the vanilla Doom II engine, but the result is reasonably faithful to the original game.
As a general note, Doom 2 feels harder than Doom. The last few levels had to be carefully planned, as it is very easy to get overwhelmed or taken off guard. In general, you need to be more cautious to deal with the monsters that were added in Doom 2 (Chaingunners, Arachnotrons, Pain Elementals, Revenants, and my personal nemesis, the Arch-vile).
Secret level MAP31 (at 01:37:15) is a tribute to id's previous game, Wolfenstein. Super secret level MAP32 (at 01:43:47) also contains four hanging Commander Keen characters in reference to another previous game from id Software.
Note that in level 3 and 5, I use a glitch to reach secret sectors. By running diagonally you jump further than when running straight.
** Completion At 01:34:46, in level 15 (Industrial Zone), I left some enemies near the end of the level because it would have taken additional time to get the blue key, go back to the exit and kill everyone, then go back to the secret exit. Apart from this exception, 100% kills achieved in all levels.
At 01:36:00, in level 15 (Industrial Zone), there's a secret sector that can't be triggered because it's too small and the player can't actually walk on it. At 03:19:00, in level 27 (Monster Condo), a teleporter pad is marked as secret, but it cannot be triggered because the player gets teleported before stepping on it. Apart from these exceptions, 100% secrets achieved in all levels.