Commonly, the Forts.exe or specific .dll files are flagged as false positives. Adding the game folder to your anti-virus whitelist often fixes "access denied" or launch errors. For those using an "Online Fix":

Click the next to your drive and select Repair Library . Address Anti-Virus Blocks :

If your game is crashing, failing to launch, or has missing files, use Steam's built-in repair tools instead of external .rar fixes. : Right-click Forts in your Steam Library. Select Properties > Installed Files (or Local Files ).