The joker and the thief are isolated, both literally and metaphorically. Literally because they seem to be traveling alone between cities in a Western or Biblical landscape with the wind and the wildcats and all the rest of it. Metaphorically because they consider themselves to be separate from the people "along the line" who think that life is a joke and who can't appreciate all the good things they use. But the city with the watchtowers is also isolated. It's enclosed by some kind of defensive wall or perimeter, which sets it apart from the surroundings. People approach it on horseback, not with cars or trains. The whole song seems to be set in a different age, when it took a really long time to get from one place to another.
Because this is a rock and roll song, we have decided not to write possible academic arguments, as it seems to detract from the heart and soul of the song. Please let us know if you disagree!