The problem could possibly be caused by the copy protection the game uses, the first thing to try would be ensure you have the latest patch for Stronghold Legends, which is currently at version 1.2 available here.
 
If the same problem occurs, the next thing would be to ensure you don't have any of following software running:
 
If you have any of the following programs running in the background, you will have to either disable them or uninstall them. This is because the latest version of the copy protection software (Securom) will refuse to run with the following programs running in the background:
 
Alcohol 120%
BlindWrite
Daemon Tools
Game-Jack
CloneCD
Game Jackal
Xoom Game Clone
CDSpace
Virtual CD
Fantom CD
Anti-Blaxx
 
The creators of Securom have created a handy diagnostic tool, which may solve the issue you are having. If the diagnostic tool fails to solve the problem. Please following the instructions on creating a analysis file. Please click here.