Installing Regnum manually is the recommended way to go. From experience, the launcher is more up to date and will cause less issues than the Steam Launcher.
Downloading Regnum Online[edit | edit source]
You can download Regnum from the official website, here.
Do not get confused by the version it displays. The "Windows XP/Vista/7" version will work fine with Windows 10 and any other newer version. Same for Linux/macOS.