It’s by far the easiest and less troublesome way to play Regnum on Linux, at the expense of a bigger install size if you’re not using Proton already.
On recent Debian and derivative systems, you can install Steam by installing the steam-installer package. This is great because you don’t need flatpaks anymore.
We assume steam is already installed, and you’ve already installed Regnum in your library

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The proton runtime will then be downloaded, and so will the Windows version of the game.
You’re done, have fun!