I have a city on a tile, adjacent to the city is a foreign settler on a plains tile, and adjacent to this settler on the hex opposite the city is my worker with 2/2 moves at the start of his turn. The foreign settler stands between my worker and city on the only flat terrain. As I understand it, my worker should be able to pass through the foreign settler and end its turn inside my city, as I have sufficient movement to do so. For some reason the game will not let me, and suggests a path through nearby forest that will take 3 turns to reach the city. Why?