As of this evening, I have finished the XBLA edition of id's 1992 classic, Wolfenstein 3D, with all 12 of the possible achievements in hand.
This was not a particularly difficult feat -- playing through the entire game on the 'Bring 'Em On' difficulty level covers most of them. Getting the rest requires searching at least one level thoroughly enough to collect all the treasures and kill all the enemies, and surviving a single level on the highest 'I Am Death Incarnate' difficulty level (which is not nearly as tough as DOOM's 'Nightmare' difficulty level.)
What slowly dawned on me as I played through the XBLA game was that I have actually never played THIS version of Wolfenstein before. I goofed around with the demo version on the PC, back in the day, of course, but my first serious playtime with it was on the Atari Jaguar. The Jaguar version featured different level maps and different bosses -- it was clearly the Wolfenstein 3D engine, but the map data was significantly different. Enemy sprites were higher-resolution, based on the Macintosh version, and additional weapons were available, like flamethrowers and rockets, that are nowhere to be found in the original. A more recent encounter with Wolfenstein came on the Game Boy Advance, but I never really tried to beat that version of the game -- it was cool to see it running well on a handheld, and it was a cheap discount bin game when I picked it up, but the GBA wasn't an ideal platform for an FPS.
So the XBLA version has been a double treat for me -- as it turned out I got to experience 55 levels that were new to me. There are actually 60 levels, with one secret tenth level hidden in each episode -- I only found my way to Level 10 while playing episode 5, so I still haven't seen ALL there is to see. But the achievements provided an excuse to play the game one more time, I've had fun, and now I'm ready to move on to (or return to) something else.