This is true. However i dont believe that is from a force field. Because you'll notice in the Lore, that there have been and are many battles along the wall defending the city. Also that it was built by titans, who died in droves constructing the wall to protect the last pieces of humanity. Also noted on other pieces of info, there are many tales of refugees making their way to the city under the traveler in the days after the collapse and how hard the trail was to get there etc.