Indeed, Castle Wolfenstein is such a veritable masterpiece that calling it "special" is a huge understatement. As its spiritual ancestor, the game's premise will be familiar to all Wolfenstein 3D: you are an allied soldier who's been taken prisoner by the Nazis. You must escape from their castle, avoiding or killing Nazi guards along the way. The action is viewed from an overhead perspective, with 2D graphics for the objects and characters (hence its nickname Wolfenstein 2D by Wolf 3D fans). While not Earth-shattering, the graphics are more than adequate, and never gets in the way of the action. There are even different graphics to depict a dead guard, a dead guard with bullet-proof vest on, and a guard shot dead at his desk... talk about rigor mortis.
Of course, graphics alone doesn't necessary make a great game, and Castle Wolfenstein has plenty of action and adrenaline rush. You start out with a gun and one 10-bullet clip, given by a fellow prisoner. More bullets can be found by searching guards and breaking into supply boxes. There is a lot of great ideas in the game, many of which are rarely used even in today's crop of 3D-accelerated games. You can, for example, hold a Nazi guard at gunpoint to search him for supplies (unfortunately, though, you can't hold the guard as hostage to facilitate your escape-- only for a short while). You can also toss grenades at guards and watch the gory effect (or as gory as 2D, 4-color CGA palette can get). The grenades can burst through (concrete) walls, and it's a lot of fun to time it so that the grenade will hit the unsuspecting SS guard on the *other side* of the wall. Evil fun :)
One thing that makes Castle Wolfenstein very replayable is the fact that the castle is randomly generated each time you play. If you are killed or captured, you can either attempt the same castle again, or play a new random one, which could very well be more impossible than the last. SS guards also tend to surprise you, thus adding to the adrenaline rush of escaping. There is really nothing I can think of that's bad about the game. Castle Wolfenstein is a true pioneer in the now-crowded field of action games, and is a revolutionary game that remains, almost 20 years after its release, eminently playable (with slowdown program, of course). Two thumbs up, wayyy up!