Back when DOOM II first came out, I had gotten my first "modern" computer, and I was between semesters at college. As I had my computer at home, I had no one at school to play against. Fortunately, a couple of my best friends still lived in the neighborhood, so late at night we would connect our computers via modem to play deathmatch with monsters.
(The atmosphere in my bedroom at the time was awesome, too. I would have headphones on, and all the lights would be out except for my bedside lamp, which had this odd plastic orange lampshade. Thus, my room would be filled with this dull orange light...)
Anyway, deathmatch with monsters was fun because not only did we have each other to beat on, we could also take out the level's monsters if they were getting in our way or if we wanted a challenge. What made it even more fun was that if you had multiple players on a level, the game would alter the monsters in the room to match the challenge. In one level ("O of Destruction", if I remember correctly), I remember spawning in, hearing "ker-THWOMP ker-THWOMP ker-THWOMP", and thinking, "That sounds like a Cyberdemon. Nah, couldn't be, there isn't one in this level." I turned a corner, and right after my character was turned to paste by three rockets, said, "Holy shit there IS a Cyberdemon!"
To this day I'm not sure who had the higher kill count that day: us or the Cyberdemon. I DO know we had to call a temporary truce so we could take out the Cyberdemon...