The solution removes the artistic side bars that appear at the menu screens, cutscenes and gameplay at 32:9 and wider, and to a lesser extent, at 21:9.
- Download the archive at the bottom of the page.
- Unpack the files to the \ExpeditionsRome\Binaries\Win64\ folder inside the game directory.
To uninstall, simply remove or move the files.
Tested on the latest GOG version at 2560x1080 and 3840x1080.
Made possible by 3Dmigoto.