Background: I first played this game on a 486 back in the 90s.
This is a great game, but if you compare it to new games, or even a Doom type game it will look very primitive. You will also be missing the point of how important this was to the history of FPS games.
The game is simple to pick up, hard to master. To master the toughest difficulty, I am Death Incarnate, you will need some good FPS skills, good choice of knowing when to use each type of weapon and some level knowledge. The game is also has a good element of skill to it. The closer you are, the more likely you are to hit as well as get hit. You can also take a lot more damage up close so you have to be careful going through doorways and in tight spaces.
There are secrets to find, treasure to pick up, par times to bet, Hitler to kill, and a ton of Nazis to mow down on the way. The game even lets you hold MORE THAN TWO GUNS AT ONCE! :P However there are only two weapons you will use long term, the grey machine gun which has great economy with the bullets and a decent rate of fire. There is also the gatling gun which never gets tiring to use. It has an amazing sound, an amazing red flash on the screen and watching your ammo quickly depleting makes you realise just how powerful it is.
For a perfectionist this game is great, at the end of each level you are rated on what % you kill, what % of the secrets you found and how much of of the nazi loot you confiscated. Back in the 90s I spent a lot of time carefully walking around every wall holding down space waiting to hear the magical sound of a new secret passage opening. The the excitement of wondering if I've found just a block of ammo, or maybe a crapload of treasure.
The game has 6 episodes, each with 9 levels + a secret level on each episode.