Bravo1853
Auto-translated
Perhaps that's the case. But I didn't find this exit, and my computer didn't lead me to it, and it usually does a good job of finding passages. He had a large army, meaning he was capable of breaking through a strong outpost, and what was likely holding him back was that the potential exit wasn't appealing to him in any way.
This is more of an author's interpretation of the situation. However, by strongly tying the color to the type of Castle, one could achieve a better atmosphere in the landscape.
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 gathered. For island maps, neutral Castles standing separately from everything create more problems than the potential benefits they offer. By the time you have an army, you face a choice: capture a neutral Castle or a Castle belonging to an opponent. It's always better to capture an opponent's Castle. By acquiring a neutral Castle in a remote location, you weaken your central Castle because the main hero can be teleported not home, but to this wasteland via the "city portal."