Okay, I want someone to PROVE to me that DOOM I & II had "hordes of enemies" in numbers greater than two dozen or so on screen at once. Everyone keeps echoing each other about this, and the Serious Sam developers keep capitalizing off the DOOM name, but I suspect that everyone is just wrong about this. It is a collective error.
Granted, the original levels themselves didn't actually have more than about 20 or so enemies at any one point (unless you consider letting that ball that shoots flaming skulls out of its mouth run around for awhile). However, Doom and Doom 2 were also about the myriad of user made levels, some of which are still some of the best levels ever made, and there were levels amongst those that had literally hundreds of imps, big tomatoes etc coming at you at the same time
A friend and I used to coop a certain remake of the original Doom 1 maps with about 400 creatures per map (over direct modem connections no less, sure made our "pay per second" phone companies happy), and those maps even brought my excellent
(at the time) 486dx2-66 to its knees.
Ah, but the sheer joy of standing there, watching some walls open, and seeing a massive horde of pinkies run at us, we kinda look at each other, stand back to back, and simultaneously start up those chainsaws.
God I loved Doom.