Since no one here has documented it, I figured this will be a great help for those who do not want to use WinROTTGL:

1) First, go to https://icculus.org/rott/

2) Then, go down to "Windows Binaries".

3) Click on the zip link where it says "Development Snapshot".

4) Extract the contents of the Icculus ROTT sourceport's zip file on your PC.

5) Download https://www.libsdl.org/release/SDL-1.2.15-win32.zip and extract SDL.DLL into the Icculus ROTT sourceport's folder as mentioned in step 4. Place this file in the same directory as all of the sourceport's .exe files shown in the sourceport's folder.

6) Now go to https://www.dll-files.com/sdl_mixer.dll.html

7) Download SDL Mixer 1.2.7.0 and extract SDL_MIXER.DLL into the sourceport's folder as mentioned in step 4. Place this file in the same directory as all of the sourceport's .exe files shown in the sourceport's folder.

8) From your ROTT GOG installation, place all of the following files into the sourceport's directory where rott-darkwar.exe is:

DARKWAR.RTC
DARKWAR.RTL
DARKWAR.WAD
REMOTE1.RTS

9) In the Icculus ROTT souceport, double-click on rott-darkwar.exe to start ROTT: Dark War.

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Edit (5/18/2020): The last revision of this sourceport is from 2015. Here are 32-bit and 64-bit windows binaries based off of the last 2015 revision:

https://lerppu.net/icculus-rott.zip

Since the SDL files are already included in the above zip, just add only the DARKWAR and REMOTE files mentioned above, then run one of the ROTT exes in the folder.

YMMV (Your Mileage May Vary)

Does Icculus support proper widescreen without stretching?

WinRottGL stretches everything in widescreen.

I can't answer that, since in all honesty, I don't know. Sadly, there are no great sourceports for ROTT: Dark War, so Icculus's port and WinROTTGL are the only ones.

Obviously, there is DOSBox as well, but that's not a sourceport.

If you know how to compile source code, Icculus' port is still being updated, so maybe more recent editions of this port do have widescreen, but you'd have to find out for yourself.

Edit (5/18/2020): The last revision of this sourceport is from 2015. Here are 32-bit and 64-bit windows binaries based off of the last 2015 revision:

https://lerppu.net/icculus-rott.zip

Since the SDL files are already included in the above zip, just add only the DARKWAR and REMOTE files mentioned above, then run one of the ROTT exes in the folder.

YMMV (Your Mileage May Vary)

Thank you for your help.

No problem.

https://www.doomworld.com/forum/topic/114535-does-rise-of-the-triad-have-any-good-modern-sourceports/?tab=comments#comment-2132128

3D Realms is gonna be making its own source port as well:

https://www.youtube.com/watch?v=QPFS99n4X2I