If people keep asking this, I don't think it has been made clear enough what the "walls" actually are. It's not like there is one mega-city inside the first wall, leaning on the second wall, and another mega city within that.
There is a huge amount of space within a given wall, and there are various villages, cities, forests, lakes, and open plains like any large geographical location. When the city in the first episode was breached, that is one tiny breach in Wall Maria into a city which is attached to the wall. When they evacuated the citizens from that city in ep2, they're not evacuating them behind the next wall, it was just a smaller supporting barrier to deter the Titans, and it didn't work because the Armored Titan destroyed it anyway. The main plan was to evacuate everyone with the ships all the way to a much further point where they go behind Wall Rose. Citizens from all the other villages and cities all around the region between Wall Maria and Wall Rose were also evacuated into the territory around Wall Rose.
Because of one tiny breach, the entire enormous region between Wall Maria and Wall Rose becomes endangered, and had to be abandoned. Within Wall Rose itself, there are also various villages, cities, forests, lakes, and open plains like any large geographical location. There's just less space because all the territory outside have now been abandoned and lost.
Levi's scouting force left Wall Rose to go into the abandoned territory between Wall Maria and Wall Rose. That is why there were cities and stuff, because they're in the territory they previously abandoned. No one can go to the territory outside of Wall Maria anymore, because people can't even get to Wall Maria at this point. There's too much ground between Wall Rose and Wall Maria, and too many Titans, since it has been 5 years.
