I love catching up on things I missed the first time around, and experiencing the mediums as close to their original state as possible, so I'm wondering if this 20th Anniversary Edition includes an option to play it with the older assets.
My understanding is it was originally an Amiga game, but it did have a DOS port. Assuming the DOS port isn't god awful, I would like to know if this release lets me play the game as it was 25 years ago.
Yes, this release lets you play the original game with all the original art assets. Actual gameplay is nearly unchanged in the updated version, I think the only difference was they changed where the checkpoints are for restarting after death (which you will do a lot...). I think those are still updated even if you play with the original assets; the game is designed so you can switch between original and new art at any time. But, having played the DOS version a lot back when it came out, I barely noticed any differences with this version.
(To be fully clear, I played the 15th Anniversary Edition, not the 20th Anniversary Edition, but I think they're quite similar [EDIT: SEE BELOW] and if you get it from GOG you get the 15th Anniversary Edition included when you buy the 20th Anniversary edition).
I also liked the new art a lot, so it's worth checking out after you played the original. Character art was done with vector graphics so it's actually the same art just at higher resolution. 2D backgrounds are redone but manage to keep the original feel very well.
EDIT: Some quick googling suggests that the 20th Anniversary Edition has a few more changes, like including the soundtrack which was composed for the original game but wasn't actually included in-game due to space limitations. Also adds difficulty levels and achievements. So, if you want to play as close to original as possible, play 15th Anniversary Edition first. This is included with your GOG purchase.