Spear of Destiny is a sequel to FPS. A very close one. So close, that you could as well call it a stand-alone expansion pack. In nutshell, it only has new levels and new bosses, otherwise you are getting same Wolfenstein 3D experience. Steam also comes with two "lost episodes" or "mission packs". They weren't made by id software themselves, and are kinda... feel like unofficial mods that Publisher made official. So yes, the package of Spear of Destiny on Steam is quite complete.
Steam version of Spear of Destiny is quite true to original. Almost nothing was changed. Aside from in-game "Apogee" being changed into "Activision', even though by now it's outdated and should be "Bethesda", but no one cares anymore. It also completely removed help section out of game, most likely due to outdated ordering info. So no intro story to be found there.
It uses DosBox to make games run on modern PCs. And as usual, the configuration of DosBox is the one that will make the game lag heavily. But it's fixed easily. Just go to game's folder and edit conf file of game for DosBox to use OpenGl rendered. It will fix lags and allow you to make screens with Steam Overlay too.
When you launch the game, you are allowed to choose between original Spear and two mission pack. One should note that they all use same save slots. Yes, you can even load saves from another pack, and it kinda works, landing you in current's pack level in saved spot! Didn't check futher though.
Spear of Destiny is a pretty simple expansion pack. It doesn't add new enemies. It doesn't add new weapons. It's same old things as Wolfenstein 3D, even if still decently done. Instead, what it has is around 20 new levels, divided into 4 generic themes, like yellow dungeon and blue corridors of blue bricks in prison part. All while staying in a single castle, working floor after floor up, to the top, where Spear of Destiny is kept, that you must to retrive from hands of Nazis, storyline being prequel to Wolfenstein 3D.
At the end of each set of levels of single theme, that is around 4-5 levels per theme, you will face a boss, that are the only new enemies in this game. ...Which require absolutely same tactics of "pop out, shoot some before they shoot back, hide back, go to secret cache of supplies if things go bad", so they are still lame. And sure thing there are new decorative objects here and there, as well as new textures here and there. It also added a bunch of new music, some of which is actually quite great, "Evil Incarnate" is quite memoriable track, played on the final.
The only memoriable, new experience got to be the final. After defeating the boss on pre-last level, you will get a key to the room with spear inside it. Once you pick it up, you will hear a short fanfare that felt like it was interrupted at the end. Yet the game doesn't end here and you are free to move. And once you open door and come out of the room, you found out that you was sneakily transported into next, last level, a Doom-styled hell, where you have to defeat THE final boss.
Oh. And it also has a proper ending, with music, credits and nice artwork, like a proper videogame. Much better than usual id software's text "oh, you won. You are the best. Wow. You married president's girl. You are such baddass!!"
The 20 levels does provide a much more packed experience than dragged out Wolfenstein 3D with 6 episodes, so it's not so all that dull and may be more fun. But it doesn't have Hitler to kill, and the core gameplay is still aged, with hitscan enemies everywhere, making you use cover tactics and "spam first", so...
And then there are two additional mission packs. Those were made by different people, who tried to also mod the game and make it feel more different. But the only thing you could change in Wolf3D before Source Code release were sprites, sounds, textures and layouts of levels. So yes, nothing new, it just got a whole new skin. A blue-green one. And feels like a cartoon, a lot, which is kinda both feels good and bad.
It also has new bosses and a new bat enemy... Nah. Just kidding. Those are reskins.
And then there are levels themselves... They weren't designed all that good. Most often, it's designed to be as complex as possible. Mission pack 2 has either the levels that will abuse your brain and are chore to beat or the ones that can be quickly skipped through. Mission pack 3 is more of the middle between those two, being a bit better designed. New textures help to set more interesting themes to it, like submarine and research labolatories, so at least this is good and brings more variety. Until you get to explore the maze where the only wall texture is a blue cave one. Repeated over and over. Such joy.
And yes, they want levels to be so complex, that in these mission packs finding secret pushwalls is mandatory to beat the level. Get ready to hump every wall. I must say, that the first theme in Mission pack 2 was cleverly designed around this and I liked it, but I felt I needed to take a good break of simple action, but it just kept on frying up my brain with mazes, so whole experience is the one to avoid. There is a good protip: secret pushwalls are marked by having unique object placed nearby.
Memoriable positive experience? Final level of Mission Pack 3, where it sends into future. With UAC crates placed around.
So yes. Mission Packs 2 and 3 should be avoided by casuals. Only for fans who want their complex tiring mazes.
Original campaign is still not bad, but as said... core gameplay remains same as Wolf3D. It didn't age.