Duke Nukem Add-ons
Here you can find a collection of levels and mods to download for Apogee's original Duke Nukem sidescroller (aka Duke Nukum).
I have tested all these level packs either to completion or up until the first point where I got stuck in a way that seemed to be a design error, but be warned that I'm providing them as-is. Some are incomplete, have bugs, or otherwise cannot be beaten; the Duke Nukem 1 level format has some peculiarities that make it easy for an amateur designer to create unintentionally broken levels if the maps are not built carefully. In cases where I'm aware of such issues I have made note of them, and on which levels if anyone is minded to make fixed versions for themselves.
Further, some of these levels must be unpacked using a tool called DE_DECOD which was intended to prevent people from running the levels on a shareware version of Duke Nukem (which is otherwise possible as the shareware edition is not missing any necessary graphics or game logic to run levels from any episode). These files have a .ENC extension and will become .ZIP files when properly decoded; instructions on decrypting them are provided in the readme files and should work in DOSBox if you can't run DOS programs natively. If you still have trouble with the unpacking procedure, contact me for help. I've noted which level packs do this, as well.
Levels by other authors
"The Returning (2017 edition)" by (_DNV_)
A complete episode with partial new graphics, including some new tiles and an updated look for Duke more in keeping with his later games (shades and a red top). Some of the enemy graphics were tweaked a bit, Duke's gun now has a plasma blaster rather than raygun look and the "confetti" from explosions was made to look more like scrapnel (which has the amusing side effect of Duke also exploding into scrapnel if he dies). The levels have top-notch visuals and are often clever and challenging; the use of fans is particularly interesting and the Dr. Proton battle is also a cool take on the fight. The new "hallway" level is also one of the best takes on an intermission level in Duke 1 that I've seen. The levels attemt a degree of realism as far as the game allows, starting Duke off in some post-apocalyptic urban ruins and eventually having him descend into a suitably technological lair of Dr. Proton inside some lava caverns. The first level has some death traps if you fall in certain pits before getting the boots, but I didn't find any place that forces an F10 level restart. There are also a few optional modules offered to tweak the experience if desired; I played the mod's default settings. Highly recommended overall.
"Dr. Proton's Return" by NicksSoftware
A complete episode with a few new graphics (mostly a red city background that appears in every level). Duke has to battle Dr. Proton once again. The levels are challenging and pretty decently designed with no glaring bugs although some of them have areas where it's difficult/impossible to avoid damage while passing through. There is usually plenty of health to find, though. The "hallway" was also peculiarly edited to have a skippable full level beyond the door that you can explore if you want to; you can potentially collect more health that way but it's a hard enough level that it's usually not a net gain.
"WARTIME 2" by Jim S. Hart
A complete set of challenging levels with trick jumps and other expert play designs, however a bug in one of the later levels (WORLDAL6) renders it impossible to beat (a locked door turns to wall rather than open space). Still one of the better sets up to that point. Uses DE_DECOD.
"Shifteds first Duke Nukem level" by The Shifted One
A single moderately sized level with a few hidden areas. No weapon or ability upgrades that I found. Fairly leisurely to play compared to most custom sets.
"DN World Test" by Videogamer555
A small single level that's nicely made and good looking, but as the title implies, more of a test than a full level; short and fairly easy.
Neil McRae's first levels
Three full episodes of Duke action. There are some tiling errors and other oddities - one level in the first episode non-fatally glitches up the game such that the text appears all messed up; I recommend saving, restarting the game and reloading after you beat that one so you don't have to go through the rest of the game with a glitched display. There are some other bugs and possible permastuck situations as well, and several times where sections of levels are identical or very similar to those in previous levels. However, there are a lot of interesting and clever ideas in this set as well. Some of the same old Dr. Proton taunts from the original game are re-used, but always chosen in a way that they remain appropriate to the new levels they appear in, which is a nice touch. There are several instances where you can fight Dr. Proton and finish the game early, or go to an exit door and continue on through other levels towards a final showdown arena instead.
Neil McRae's second levels
Neil's second set of levels contains one more episode much like the previous ones, with reused themes and a few tiling problems especially around elevators. There are also several levels in this set which are unfinished and have no enemies. Like the previous episodes, it's possible to skip fighting Dr. Proton, but this time I advise you don't... there's one more vacant unfinished level after that and then I ended up in a weird all-grey invisible walls level which I gave up on after a while. Weirdly, I couldn't find any such level in the actual level data so I can only assume that something about this set caused the game to glitch out. Can't really recommend this one.
Pieter Simoons (Radiant)'s levels
A couple of levels built around the brick-themed tiles. The first one is fairly basic but it's possible to get yourself permastuck if you do things wrong. The second level is very difficult, as there's no health until the end and a lot of dynamite boxes blocking you from shooting enemies and other tricky situations. The "hallway" level was also edited, with an odd sort of museum area that you can look at if you walk past the door. Do not enter the second level while facing left in the hallway, though, or you'll miss the crucial first key and have to restart the map.
DUKE_DB1 by David Bollinger - DukeEdit v2.0 pack-in
A well-made set with good tile usage. It is challenging as health is metered out and often with sodas rather than turkeys so you have to be a bit careful with your shooting. Uses DE_DECOD.
DUKE_DB2 by David Bollinger - DukeEdit v2.0 pack-in
A more experimental set with some maze levels and also a few odd excursions into implementing secret passages in the levels. It's an interesting trick but I have mixed feelings about it; Duke wasn't really meant to have such passages so it comes off as a little bit hacky. The set is incomplete and eventually you will come to a place where you're trapped with an impossible jump because the platform is too high (WORLDAL7). Consider it a "win" when you get there although there is one more incomplete level after it. Uses DE_DECOD.
"DUKE'S NEXT ADVENTURE" by Larry Shanker (DUKE_LS1) - DukeEdit v2.0 pack-in
The levels start off bland but improve some as the set goes on; probably the author was learning along the way and didn't throw out the first attempts. The visuals have a lot of ugly tiling mistakes and it's possible in some places to get yourself stuck if you're a little too eager to blow up the destructible bricks. Uses DE_DECOD.
DUKE_LS2 by Larry Shanker - DukeEdit v2.0 pack-in
Although better than the previous set visually, there's still some off tiling. There is also a brick-breaking themed level that can feel very tedious and I ended up coming to a level that seemed to require the grapple ability which I hadn't yet found. Uses DE_DECOD.
"RAMBO levels" by William Witkowski (DUKE_WW1) - DukeEdit v2.0 pack-in
High-octane levels with lots of enemies but also lots of health, and gives you gun powerups early. However, I already reached an inescapable situation in level 2 (WORLDAL3) - the purple key is stuck in the wall and can't be collected. Uses DE_DECOD.