Decided to jump right into Doom 2. Released about 10 month after Doom 1 (impressive!) Doom 2 moved from mail order to a regular box release, and has 32 levels. There's a fair number of smallish engine improvements (and some more creative combinations/execution of existing tricks), most of which have filtered into the later Doom 1 level sets that I already played.
Over time, I think it's become more or less accepted that the Doom 1 levels are generally better than the Doom 2 ones. I forgot how much of a slog a lot of these early levels are, just kind of boring and basic. Even compared to Episode 1, the first five are pretty small (E1M1 is simple, but carefully executed, Level 1 of Doom 2 is just small and awkward). On the other hand, Doom 2 really throws the guns at you, giving you everything up the the plasma gun by the end of level five (first appearance E2M1, the 10th level, in original Doom). The first level gives you three weapons (shotgun, but not much ammo, chainsaw, and the ROCKET LAUNCHER!) and the second adds the Super Shotgun.
Ironically, it's not actually that great in these early levels. Against a crowd of sergeants (the biggest threat at this point in the game) it's possible to line up a great SSG shot and get all of them, but you'll often leave one or more. The shotgun will usually clear the group in two shots, but a much shorter reload, leaving you less open to retaliation.
Level three (The Ga(u)ntlet, most Doom 2 levels are THE something) adds the chaingunner, the first new enemy. They're tougher than shotgunners, and can deal more damage if they zero in, but don't have the burst or sniping potential. It does have a great hidden spectre in a dark pit. They're much more invisible in darker areas, and Doom 2 takes advantage of this more than Doom 1.
Level four has a light switch. That's about the most exciting thing I can say about the level.
Hell Knights (Barons with half health and less color) debut in level five. It also has the most annoying jump in all of the iD levels to get to a secret with a couple ammo boxes.
The Crusher (level 6) is the first "good" level in Doom 2 (it starts a pretty solid run of about a half dozen, mostly designed by my favorite Sandy Petersen). If you're on a high enough difficulty, your first shot will trigger the alert sound for a Spider Mastermind, the final boss of the original Doom. It's in the next room over, and not really a threat, but playing a final boss sound effect on level 6 is a fun choice. In the end, it's easily killed by the gimmick (a switch operated crushing ceiling, hence the name), but still cool. It does run a little long, with the crusher being in the second room, which you'll clear about a third of the way through the overall level. It also introduces the one new power up for Doom 2, the Megasphere. It maxes out your health and armor, and mostly just saves designers from having to stack a Berserk, Soulsphere, and Combat Armor next to each other.
Dead Simple is the first "boss" level, and opens with a text screen similar to the end of the episodes of Doom 1, which explains that you've fought your way to the middle of a space port. If you can disable the defenses, humanity will be able to evacuate the Hell-occupied Earth. Also, a backpack stacked on top of other ammo pick ups to make it REALLY full of ammo. The square "layered arena" (with multiple gangs of mid level monsters in each) will become one of the basic Doom level templates. It adds the Mancubus (XL demon w/ flamethrowers) and Arachnotron (XS Spider Demon w/ plasma gun) to the bestiary.
I'm going to wrap this up with a quick look at the changes to the bestiary and armory between Doom and Doom 2.
Doom 1's armory:
1 (Berserk)Fist/Chainsaw: Generic crappy melee weapon, better (but still situational at best) fast melee weapon, and gimmicky power up weapon.
2 Pistol: As crappy as you can get and still technically be a gun.
3 Shotgun: All around workhorse. Good power, but slowish reload
4 Chaingun: High DPS, not a lot of ammo. Great for cutting through a horde of weak monsters or locking down a small number of middish ones.
5 Rocket Launcher: Heavy splash damage
6 Plasma Gun: It's a better chaingun.
7 BFG: Slightly gimmicky ultimate weapon, almost a screen clear when used correctly.
Doom 1's bestiary:
Left-right, slight threat increase
New row, larger increase
Pistol Zombie(as weak as you can get and still be an enemy)
Imp, Shotgun Zombie (that hitscan burst is way better than a slow fireball, but slightly weaker and won't attack as fast in melee)
Demons, Spectres, Lost Souls (weaker, but makes up for it with mobility)
Cacodemon
Baron of Hell
(This space left blank)
Spider-Mastermind, Cyberdemon
Doom 2's Armory:
Only adds one weapon (the Super Shot Gun) but I'd rate it above the rocket launcher (similar damage, but no danger of blowing yourself up and more likely to flinch, but slower/less accurate) and slightly below the Plasma Gun (worse accuracy and lower DPS).
It also does ~3 shells worth of damage for the price of 2, improving your ammo efficiency considerably. The regular shotgun does still have some use: a higher rate of fire is useful for modest sizes groups of weak enemies and a tighter long range spread. Overall the SSG is good against mid tier enemies and very tight groups of weak ones. It's perfect for knocking out Demons (who want to clump up, and can reliably be one shotted, compared to two to three shots from the basic shotty).
There's also a lot more bullets in Doom 2 (partially from more plentiful ammo in general, partially since the chaingunners drop 10 rounds per kill) which makes the chaingun more reliable for situation uses against Cacodemons and other weak-but-spongy enemies. Therefore the new bestiary looks more like
Doom 2's Bestiary:
Pistol Zombie
Imp, Demon, Spectre, Lost Soul (the SSG really put in work here), Nazi, Chaingun Zombie (tougher than shotgunners but too slow/inaccurate to be that dangerous) Shotgun Zombie
Hell Knight (hits hard, but only two or three SSG blasts to drop), Cacodemon (effectively dropping a threat level due to better chaingun access), Revenant, Arachnotron, Pain Elemental, Mancubus (not much tougher than other enemies in this tier, but hits a lot harder)
(This space no longer left blank due to the filled out middle ranks and more plasma to go around) Spider-Mastermind, Arch-vile (much squishier, but more deadly and with a pain in the but special ability and quicker), Cyberdemon
Basically, iD had to really fill in those middle ranks to give you something to hit with all those SSG pellets, and the existing middle ranks became a lot more manageable. It's amazing how much one gun (and a bit more ammo) changes the whole combat balance of the game.
More Doom tomorrow! (Probably)