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Just a small clarification: the Garrison is located between the red and blue areas, and the opponent is a Barbarian, so this Garrison cannot be between grassy respawn points, which means the path is clear.
If we were to make the task more difficult and the opponent also had a grassy castle, and the Barbarian's territory became grass, then I don't think anyone could give an answer, but we would need to make it even more complicated (a hidden monastery is visible) - then both players would be playing as Castle. Now that's a real challenge!
P.S. You don't even need to look at the save file.