"Marcus makes a long journey to the city."
Translation:Marcus iter longum ad urbem facit.
If Marcus lived close to the city, "in urbem" would make sense, but if he lives some distance away, it would not make sense. I think it is understood that once he reaches the city he will enter it, just as it is understood that if I say that I am making a journey to a particular location that I will enter it.
Thanks! This looks like a great resource I will have to begin using it. Doing a bit more review I think I am incorrect to consider "city" an indirect object, rather I should have called it object of a prepositional phrase--if we were to give this any case, I think it would be Instrumental, which I haven't seen as part of Latin.