In Book 3, there is a duel between Alexandros (also known as Paris) and Menelaos. The duel happens in an area in the open space between the armies of the Trojans and the Achaians.
At the beginning of Book 3, the two armies are about to fight a battle. As they march rapidly towards one another, Paris comes out of the Trojan side as their champion to challenge any Achaian to a duel. Menelaos accepts the challenge. After some negotiations between the leaders of the two sides, a space is measured out and the two men fight in that area between the two armies.