If your game is displayed incorrectly and doesn't fill the whole screen. This problem may be caused by Large (150%) DPI settings.
You can find these here -
Control Panel\Appearance and Personalization\Display
You can find these here -
Control Panel\Appearance and Personalization\Display
To resolve the problem, do the following -
Locate the exe - C:\Program Files (x86)\Steam\steamapps\common\Stronghold Crusader Extreme
Right click on the Stronghold Crusader Extreme.exe file.
Click on the Compatibility tab.
Tick 'Display display scaling on high DPI settings'
Click on OK.
Right click on the Stronghold Crusader Extreme.exe file.
Click on the Compatibility tab.
Tick 'Display display scaling on high DPI settings'
Click on OK.