Troy is based off the ancient Greek myths of the Trojan War, the most well-known portion of which is detailed in the Homeric epic The Iliad. Despite this it took many liberties from the source myth. Some may have been for limits of time, or to modernize the story, but most seem to have been for stylistic reasons or to doubly emphasize Achilles’s role as the hero.
In The Iliad itself Achilles is absent for much of the movie, staying behind at his ships simply watching the war because Agamemnon took his trophy, a Trojan woman related to the royal family. This remains true in Troy and the difference is not one done to Achilles himself but the supporting cast. Ajax, who dueled Hector to a standstill when the Trojans attacked the Greek ships, is killed by Hector in the preceding battle. Ajax, barely mentioned by name in the movie and never by his full name, is still presented as one of the Greek’s main warriors being presented by Agamemnon’s side on several occasions. His role is minimized, and his battle against Hector the most interesting fight in The Iliad in my opinion is made into a one-sided battle to establish how powerful Hector is. This makes Achilles’s role more important both by minimizing competition for the role of hero and by re-emphasizing the skill of his main opponent. In this right here is shown a fundamental difference between The Iliad and Troy; The Iliad is the story of the Greek heroes who fought the Trojan war and Troy is a story of Achilles.
While Ajax was minimalized, his role reduced to defeating extras to make the antagonist seem stronger and the antagonist’s defeat more meaningful, other characters important to The Iliad were removed entirely or reduced to cameos. Aeneas, the second greatest warrior of the Trojan forces, is reduced to a role where his cousin Paris did not even recognize him. This was possibly done for time restraints, The Iliad is an epic and a long read and Troy simply cannot afford to detail all the characters of the story. Instead they use his bit appearance to work in a reference to the later Roman work of The Aeneid, having Paris not recognize him so that we the viewers can be informed that this is Aeneas mythic ancestor of the Julian emperors. Another reason for the minimization of Aeneas is the removal of Diomedes.
Diomedes could arguably be called the major hero for half of The Iliad. While Achilles is sulking at his ships, Diomedes is the primary Greek warrior who forces Hector to retreat and defeats Aeneas who only survives due to divine intervention, this is Aeneas’s main battle in The Iliad proper, and even injures Ares the god of war himself. From the time Achilles retreats from battle till Hector leads the assault on the Greek ships Diomedes is the main protagonist of the story, in Troy Diomedes is completely absent.
The other characters glaringly missing are the gods. In the Greek myth the gods play a major role at every step. Examples abound. Apollo strikes the Greeks with plague after Agamemnon rapes the daughter of his priest, forcing Agamemnon to give up his prize which is what instigates him to take Achilles’s and Achilles’s removal from the war. Ares enters battle for the Trojans almost routing the Greeks before Athena guides Diomedes into striking him with a spear with such accuracy and power that even the god bleeds and retreats. Poseidon often intervenes on the side of the Greeks, and in some versions of the Trojan Horse story is instrumental in the Trojans failing for it as he sends a sea monster to eat the dissenters, and Aphrodite on the side of the Trojans, and both save Aeneas at some point. Even Zeus, king of the Greek gods, takes a side at Hera’s needling declaring that the Greeks will win. This brings us to the second critically important difference. The Iliad is a story of Greek mythology and despite being a retelling of the heroic feats of men it is also a story of the gods, Troy is a story of men an epic history that could have been. Gods have no overt place in Troy, and this strikes home when dealing with Achilles for his divine heritage is an important defining factor in The Iliad and other legends.
The legend of Achilles’s imperviousness is referenced in one of the first scenes where a boy asks him:
Messenger boy: Are the stories about you true? They say your mother is an immortal goddess. They say you can’t be killed.
Achilles: Then I wouldn’t be bothering with a shield, now would I?
This scene helps establish that Achilles is not immortal, not invulnerable, but instead a man, a mortal, and most importantly vulnerable, coupled with his death where a single arrow is left in his heel (screenshot) serves as potential fuel for his legendary weakspot. This change makes him a much more interesting hero, easier for us to associate ourselves with, and makes the conflicts more engaging as there is a chance that Achilles is wounded and defeated.
These changes serve to engage us in the story of Achilles giving us a journey through epic legend which is both engaging on a human level, exciting on a cinematographic level, and concentrated enough that one can enjoy it in a day instead of dedicating nights on end to its telling.