If you’re facing the "APK not installed" issue while trying to install Castle APK, it could be due to several reasons. First, ensure your device has enough storage space. Next, check that the APK file is not corrupted by downloading it again from a trusted source. Also, make sure that "Install unknown apps" is enabled for the browser or file manager you’re using. If the APK’s version isn’t compatible with your device, download a version that matches your Android OS. Lastly, avoid conflicts by uninstalling any previous versions of the Castle app before installing the new one.