Twilight Archipelago

As far as I know, you can exit from it.

Perhaps that is the case. But, I didn't find this exit, my AI didn't go there, and it usually knows well where there is a passage. It had a large army, i.e., it was capable of breaking through a strong outpost, and what kept it in place was most likely that the potential exit was not interesting to it in any way.

The garrison between the blue and red is not that strong.

The garrison's strength stands out noticeably from the rest of the guards. This can be understood from the words of Rambo1969

not a single serious battle except for the assault on the outpost between the red and blue.

The AI's logic is similar to the flow of a liquid - it moves where it is easier. In this case: the shipyard and seafaring are the only option for it.

the maximum I can do is assign the home castles to specific locations.

This is already the author's vision of the situation. However, by rigidly linking the color to the type of castle, it would be possible to achieve a better atmosphere in the landscape.

what is a useful neutral castle?

The usefulness of a castle is always very situational. In the most general case: there is a good path to it, it can be held, and troops can be taken from it. For island maps, neutral castles standing separately from everyone create more problems than the potential benefits. By the time you already have an army, you face a choice: what to capture, a neutral castle or an enemy castle. It is always better to capture an enemy castle. By acquiring a neutral castle in a remote location, you weaken your central castle. Because the hero can be teleported not home, but to this wasteland, via a "city portal."