Fortress Destroyer has become quite tricky to actually play, with Unity’s web player being deprecated. Here are some instructions based on what worked for me (a long time ago now — even this may not work any more!):
Use the control panel to uninstall any Unity web players you might have accumulated while trying to run this game.
Install an old 32-bit Firefox
The 32-bit is non-negotiable, but how old I am not sure — too early and the NK login won’t be able to use captcha, too late and Unity won’t be supported. Version 50 definitely works; feel free to check a more recent one and let me know in the Ninja Kiwi Discord server if it works so I can improve these instructions.
Such an old version should not be used for everyday browsing. If Firefox is already your preferred browser, don’t worry: you can continue using a 64-bit version alongside this one, or you could consider using a separate Firefox-based browser such as Waterfox.
Go to about:preferences#advanced and select “never check for updates”.
This is very important, because left to its own devices, Firefox will automatically update to a 64-bit version if your computer supports it.
To be safe (or if you want a browser to play SAS4 on), go to about:addons, click the cog button at the top of the plugins section, and ensure that “update add-ons automatically” is NOT ticked.
Install Unity web player version 5.1
Do NOT go to unity3d.com/webplayer as the “Get Unity” button suggests — this page only provides you with version 4 of the web player. There are certain changes between versions 4 and 5, and even between the sub-versions, causing some games to only work on specific versions. For FD, this means we need 5.1.4