The first thing to try would be to ensure your graphics card drivers are update to the latest version available.

 

Second, ensure the game is patched to the latest version available - Version 1.4.1.

Third, ensure the graphic settings for Stronghold 2 are set to maximum in the graphics configuration panel.  You can access them by going to: Start\All Programs\Firefly Studios\Stronghold 2\Stronghold 2.