So you do use Galaxy! Now, I wouldn't ask you to uninstall the client, then try and see what happens. I don't own Sengoku so I can't test it, but maybe someone else who owns it, but doesn't use or has Galaxy installed, could play judge and jury in this case for us to decide? ;-)

With Galaxy not running the game will need you to login to your Slitherine account to go multiplayer.

The same with Galaxy running. Why it should it be more draconian than the Steam or Slitherine version?

Because they are and I could name many games with Multilplayer component where there is no way around using Galaxy to play Online. In case of Slitherine there is the account you also need. There is even a special message popping up in some of their games telling you that: You need a GOG and Slitherine account to play. GOG, for Galaxy, of course.

What I do not know is, whether the fact that Galaxy is present, but shut-down, completely, not running hidden in your taskbar or no services of it running, whether this still allows you to join a multiplayer campaign. Or to even click on multiplayer anymore to log-in that is. Or, say, if you install it without Galaxy and offline installer files, whether Galaxy and/or the lock in-game preventing you from even clicking on that MP button, would be unlocked and MP sessions (async or no) becomes possible. I doubt that that is the case but stranger things happen. :-)

Update: I asked a friend who owns Sengoku but doesn't use Galaxy. He confirmed that even completely without Galaxy but with Slitherine account can you join a multiplayer session. So, this is another exception of the common rule, noted and will add it on my list.

Talking about Fantasy General II according to the PCgamingwiki page:

https://www.pcgamingwiki.com/wiki/Fantasy_General_II

To go multiplayer requires a GOG Galaxy OR Slitherine login. The game lets you choose which one you want. Obviously without Galaxy your choose should be login via your Slitherine account.

In Field of Glory II you need just a Slitherine account. Like in the older titles (Sengoku Jidai)

And specifically in Field of Glory II Medieval I ignore it. But I guess that in the newest Slitherine Titles like Fantasy General II (Medieval could be in the list) the multipayer version gives you two options to login to the multiplayer system.

Then there is something wrong because it is advertised even in the manual:

"Multiplayer: Begin a scenario where you play against another
human. You will be prompted to login either through your GOG,
Steam or Slitherine account and then taken to a multiplayer
lobby screen."

Open a ticket in GOG just for information. Or other game owners could inform if It could be a bug.

No, see? This isn't a bug. This is the same in games like Grim Dawn, Offworld Trading Company, which is a special candidate in itself in that not even the Single Player Online Challenges can be accessed without Galaxy, not to mention this game's dedicated multiplayer mode. It is always the same: MP button is grayed out and the message says You need Galaxy.

Lan, in case there is one, or offline Co-Op, this always works, without Galaxy of course or any special account. No Galaxy no Online fun. ;-)

Well, if that is true, you have a point. I consider it should be clearly advertised if a multiplayer game mode needs Galaxy or not.

Anyway, and coming back to the beginning, you have noticed that not every multiplayer feature in GOG games needs Galaxy. After all it is the origin of all this series of posts.

I was aware of that of course. There are games on here with multiplayer online feature enabled and accessible where GOG isn't mandatory while others you would find this bit of information only when someone asks for it. I can't say anymore to this that what I already said.

All we can say after our little exchange is that I wouldn't bet on MP without Galaxy for non-Galaxy users in this one. And with this buenas noches y dulces sueños. ;-)

Played last night, had a blast correcting some history :)
Whats with strategy games and tinee-tiny writing?

another case of "if only the game looked anything like the trailer"

If this game is no different than the previous Field of Glory 2, then you won't be needing GOG Galaxy to play the multiplayer. You just need to have a Slitherine account in order to play the multiplayer.

Too true, though the Menu elements have that same feel... to a degree.
I would love if they used Medieval style painting instead of 3d models.

Question : Are the Field of Glory II DLCs compatible with this game ?

It's an independent game (although using the same engine), so why would they be?