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The first time, I didn't rush to garrison – I wandered around the newly captured turquoise castle, and an unpleasant incident occurred in the form of a red player with a sizable army. However, since Frida's starting troops were almost unscathed, we repelled the attack, but unfortunately, we no longer had enough troops to garrison.
The second time, not wanting to repeat the same mistakes, I traversed the map like a shooting star and captured a garrison with my starting army, albeit with significant losses. But it turned out that there wasn't much point in the acquisition – the turquoise player didn't have a particularly strong army, unlike the red player. It became clear that Frida (after losing the garrison) wouldn't survive an encounter with the red player, nor would the garrisons of the castles that had fallen into my hands.
I don't feel like starting a third time yet.