Baseline (Widescreen) Solution & Issues
Native. The game is a combination of 16:9 anamorphism and Hor + behavior in widescreen resolutions. All aspect ratios narrower than 16:9 use letterboxing to maintain a 16:9 aspect ratio and all aspect ratios wider than 16:9 use a combination of Hor + behavior and pillarboxing.
There are some fixed-camera events in the game that have pillarboxing in aspect ratios wider than 16:9. A video link of the behavior is provided below.
Black Bars FIX: Depending on aspect ratio copy all content from this archive - Assassins_Creed_II_BlackBars_FIX.7z. Fix available from the PC Gaming Wiki Community Files (search there).
Ultra/Super-Wide (21:9/32:9) Specific Solution & Issues
Black Bars FIX: Depending on aspect ratio copy all content from this archive - Assassins_Creed_II_BlackBars_FIX.7z. Fix available from the PC Gaming Wiki Community Files (search there).
Eyefinity / Surround Specific Solution & Issues
4k UHD Specific Solution & Issues
Stereoscopic-3D Solution & Issues