You'd think that the the Trojans would be better off sending a small force (along with Paris) to meet the Greeks outside the walls at a neutral location. If they anticipated a fight breaking out, they would have stayed inside the walls and prepared to defend using the high ground, rather than dying by the scores potentially trying to use their bodies as meat shields to guard the stone inanimate walls that are meant for their protection.
However, there are some complicated factors here, when it comes to calling this scenario historically inaccurate. One, it's a parley. Maybe the Trojans felt having the army outside the walls was necessary in order to protect Paris as a deterrent against the Greeks simply killing him?
I'm simply ignorant of how this would work out in a real-life scenario. Is the movie's depiction of the duel and the ensuing battle historically inauthentic because the Trojans would never fight outside their walls, as some have claimed? Or would an army have had a motive to plant their feet outside the walls, if only to ensure the duel went down smoothly? Thanks!
I'm really not sure what historians think of the authenticity of the Illiad but in the Illiad, major parts of the story and battle occur with the Trojan army outside the gates and fighting on the plains. In fact, there's multiple references to the two sides lining up and then taking off their armor and sitting down while the leaders negotiate or various duels take place, etc. So regardless of historical authenticity, the movie was actually fairly true to the book in that respect and that's likely what they were trying to portray. I tried to see if the Trojans give any reason for meeting them on the plains rather than fighting from the walls but all it says is that the goddess Iris goes and speaks to king Priam and Hector and tells them to assemble their men to meet the Greeks and Hector just instantly goes and does it. To quote, "Thus she spoke, but Hector knew that it was the goddess, and at once broke up the assembly. The men flew to arms; all the gates were opened, and the people thronged through them, horse and foot, with the tramp as of a great multitude." (words may vary slightly depending on translation, of course). So, to me that seems to imply that there never was much question as to whether they would fight from the walls or on the plains. It's just assumed by the Trojans that if you fight, you fight face to face. Again, I think you need to keep 3 possible versions straight: there's what happened in the movie, what happened in the actual text and what could have happened historically. I'm pretty sure the movie is based purely on the text and the way they showed it is fairly accurate to the text.
Edit: I should also note that in the book, that duel ends with Aphrodite dragging Paris to safety in Troy. I'm not sure that really has any relevance but the actual text has tons of interactions and interventions from the various gods and godesses. Take that how you will.