Steam Error Code 51 typically indicates an unexpected communication block or interception between the Steam client and the game executable. This failure is heavily driven by third-party background software or security suites mistaking the initial game launch handshake for an unauthorised process, causing Steam to abort the startup sequence immediately.

Please try the following troubleshooting steps to clear this interference and resolve the issue:

1. Configure Antivirus and Security Exceptions
Third-party security software, particularly protective modules like F-Secure DeepGuard, Avast, AVG, or Windows Defender, are the most frequent cause of Error Code 51. Please add the entire Stronghold: Definitive Edition installation folder or the main game executable file as an explicit exception within your security software settings to prevent false positives from halting the engine.

2. Verify Integrity of Game Files
A corrupted or incomplete file download can easily disrupt the initial startup sequence. In your Steam Library, right-click on Stronghold: Definitive Edition and select Properties. Navigate to the Installed Files tab and click on the Verify integrity of game files button to allow Steam to scan, repair, and replace any broken data.

3. Close Conflicting Background Applications and Overlays
Software that injects custom overlays or hooks directly into active game windows can conflict with the Steam framework during launch. Completely shut down applications like MSI Afterburner, RivaTuner Statistics Server, or specialised peripheral management software before attempting to launch the game.

4. Update Graphics Card Drivers
An outdated display driver can cause an immediate initialisation failure right as the engine tries to establish a graphical canvas. Ensure you have downloaded and installed the latest official driver package directly from your hardware manufacturer's website to ensure compatibility.