This problem seems to be related to dual monitors.

Without disabling the second monitor in the Windows desktop. Right-click on the Stronghold2.exe file, click on Properties, Compatibility, tick on "Disable visual themes" and "Disable desktop composition".

Otherwise, disable the second monitor and try running the game as normal. Right click on the desktop, Click on Screen resolution, in the multiple displays click on show only desktop 1 or 2 depending on which is your main monitor.