Gaius of the Julii, called Caesar, simplified as Gaius Julius Caesar, is a central character in Rome played by Ciarán Hinds. His conflict with Pompey forms the first major story arc. He is based on the historical Gaius Julius Caesar.
Caesar is a man of both military and political talent, charismatic and shrewd, calculating and deliberate. He usually maintains a calm demeanour, though has been known to enter into rages when pushed too far. He is a forgiving man, readily granting amnesty to everyone of Pompey's faction who deserts to his cause or surrenders to him after Pompey's defeat. He attempts to make life fairer for the plebeians of Rome, though this angers the patrician eltie and is ultimately his downfall. Anxious not to appear as a tyrant or a king, Caesar is nonetheless not above using dark methods to further his ends, arranging the assassinations of certain critics of his as well as rigging elections to ensure a favourable outcome for himself. Is very adept at kicking ass when in war.
At the beginning of Season One, Caesar has just won an eight-year war of conquest in Gaul, and accepts the surrender of Vercingetorix, the Gallic leader, after the battle of Alesia. On the same day, however, he receives news of his daughter Julia's death, and attempts to marry another of the women in his family to Pompey, Julia's widower, in order to maintain their alliance. This is to no avail, however, as Pompey marries Cornelia, the daughter of Caesar's outspoken critic Scipio. Immediately after Alesia, the eagle standard of Legio XIII Gemina, one of Caesar's legions and the legion of Lucius Vorenus and Titus Pullo, is stolen. Caesar tasks Mark Antony, who in turn tasks Lucius Vorenus, with finding the eagle. Not expecting that they will succeed, Caesar in fact hopes that the eagle will remain lost, tricking Pompey into believing that Caesar is weak and underestimating him. However, Vorenus and Pullo, as well as Caesar's great-nephew Octavian, return with his eagle and reveal Pompey's involvement in the eagle's theft. Caesar marches back toward Italy, and winters the Thirteenth at Ravenna.
Caesar sends Mark Antony ahead to Rome in order to take up the office of Tribune of the Plebs with orders to ensure that Caesar will be able to be spared a criminal trial in Rome by his enemies. However, Antony fails when a mob of Pompey's men defy his orders and attack Antony and his escort of Thirteenth men, including Vorenus and Pullo. Antony is forced to flee the city and the declaration of Caesar's criminality is passed. Caesar, sensing another opportunity Caesar incites the Thirteenth to willingly cross the river Rubicon armed, making themselves rebels and traitors, and march on Rome. By moving quickly Caesar is able to catch Pompey off guard and the older man is forced to abandon Rome, which Caesar takes without resistance. After resting in Rome temporarily, and being forced to abandon his long-time mistress Servilia of the Junii in order to maintain his political marriage to Calpurnia, Caesar leaves the city, leaving Antony and the Thirteenth behind to maintain order, and pursues Pompey to Greece, where he loses the first battles to Pompey's forces. With defeat now a very real possibility, he summons Antony and the Thirteenth to reinforce him. After Pompey unnecessarily gives battle to at Pharsalus, Caesar crushes Pompey's forces and later accepts the surrender of Marcus Junius Brutus and Marcus Tullius Cicero, who had both joined Pompey's faction at the beginning of the civil war. Caesar readily forgives both men, but is saddened to learn that Pompey has no intention of surrendering, and has instead fled with his family and a small entourage, and that Scipio and Cato are determined to continue the war in Africa. Caesar learns from Vorenus and Pullo, who coincidentally ran into Pompey after his defeat, that Pompey has gone to Egypt and follows him, hoping to accept his surrender.
Upon arriving in Egypt, Caesar meets with the boy king Ptolemy XIII, whom he finds arrogant and insufferable. He learns that Ptolemy's sister-wife, Cleopatra, has challenged his rule and is being held under house arrest. Hoping to win Caesar's support, Ptolemy and his courtiers tell him that they have had Pompey murdered and bring Caesar Pompey's severed head, imagining that he would be pleased at the death of his enemy. Caesar, however, is angered and upset by the revelation and demands that the Egyptians hand the killer over to him. He gives Pompey's head a Roman funeral, and weeps as it burns on the funeral pyre. Knowing that the internal strife in Egypt has the potential to disrupt the grain supply to Rome, with dire consequences, Caesar sends Antony and half of the Thirteenth back to Rome while he remains in Egypt with the other half, intending to arbitrate the dispute between Ptolemy and Cleopatra rather than fight. The next day Caesar orders Vorenus and Pullo to find Cleopatra and bring her to him, while he brings the Egyptians to heel. He calls in debts owed to Rome by Egypt, at which Ptolemy becomes angered and dismissive of Caesar. Caesar reminds the boy king that he is a vassal of Rome, which further enrages the young monarch. However, as he is beginning to argue Caesar cows him into compliance. Deciding to placate Caesar for the time being, the Egyptians concede to his demand for Pompey's assassin, who turns out to be a Roman named Septimius. Caesar has Septimius executed and his head placed on a spike outside the palace.
Soon afterwards, Vorenus and Pullo return, having smuggled Cleopatra to Caesar by rolling her up in a carpet. Upon seeing her, Caesar seems taken by her beauty and demure manner. Caesar and Cleopatra come before Ptolemy and swiftly take control of Egypt. After being accused of influencing young Ptolemy's decision to plot against his siter-wife , Ptolemy's courtiers are executed and their heads join Septimius' on spikes outside the palace . Angered at this display of Roman imperialism, slowly an Egyptian mob gathers outside the gates . Meanwhile, Cleopatra secures an alliance with Caesar by sleeping with him and instilling in him the importance of having a male heir to carry on his legacy. As the angry mob grows the Roman soldiers prepare battle formations against the impending revolt. Almost a year later, Caesar has won in Alexandria and proudly presents an infant boy to his men, whom nearly all present believe to be Caesar's son. The only ones who think otherwise are Vorenus and Pullo, who both know that the child, Caesarion, is Pullo's son.
The civil war ends with the defeat and suicides of Scipio and Cato, and Caesar returns to Rome. At a dinner held by Atia to welcome both Caesar and Octavian, who had spent the previous two years in Mediolanium, Caesar asks Octavian how he would go about reforming the Republic. Octavian's thoughtful response impresses Caesar, who appoints his great-nephew to the position of pontiff, despite the protests of the Chief Augur, Servilia and Octavian himself. Caesar overrules all of them, however, and insists on Octavian taking the position. Next, Caesar visits the house of Lucius Vorenus, and asks him to stand for the magistrace of Lower Aventine on Caesar's slate. Vorenus initially refuses, but Caesar and Vorenus' wife, Niobe persuade him to accept. Caesar presents Vorenus to a cheering crowd.
The Senate soon grants Caesar the office of Dictator, and Caesar declares that the civil war is finally over and that there will be five days of games and feasts in honour of his triumph. On the day of the triumph Caesar is annointed by Octavian, now a pontiff, who paints his face with ox blood. After a parade of the Thirteenth Legion, Caesar orders the execution of Vercingetorix, who has been his prisoner since Caesar's victory at Alesia seven years before. Despite his popularity with the people, however, and his forgiveness of his former enemies, a damning pamphlet of Caesar circulates throughout Rome. The pamphlet's author is, ostensibly, Brutus (though unbeknownst to almost everyone the true authors are Servilia, Cassius and Quintus Pompey). Brutus assures Caesar that he had nothing to do with the pamphlet, though he does not divulge the names of its true authors, and Caesar readily believes him, though he still wonders who wrote it.
Having rigged the magisterial elections, Caesar's candidates win throughout the city, including Vorenus. Vorenus comes to Caesar after he receives a complaint from another of Caesar's veterans of Gaul, Mascius, who tells Vorenus that he and the other veterans wants land in Italy. After being informed of his veterans' dissatisfaction Caesar wonders whether they might turn on him. Vorenus assures him that they would never take up arms against Caesar but would turn to banditry, in Caesar's name. Caesar learns from Vorenus the extent of Mascius' influence among the veterans and orders Vorenus to offer Mascius land in Pannonia, at the edge of the empire near Germania, and to bribe Mascius into accepting if, at first, he refuses. Caesar leaves it to Vorenus to decide how much money to offer Mascius. Meanwhile, graffiti has appeared throughout the city that depicts Brutus murdering Caesar. At a symposium at Atia's house, she pulls Caesar to one side and warns her uncle about Brutus, telling him that Servilia will not rest until Caesar is dead. Caesar dismisses her warnings, however, and believes she is simply being dramatic. Vorenus and Niobe, whom Caesar has invited as his special guests, arrive and Vorenus and Caesar fall into discussion about the fate of Titus Pullo, who is soon to be tried for murder. Octavian, who is fond of Pullo, pleads with his great-uncle to help Pullo but Caesar refuses, saying that Pullo's victim was Aufidius Dento, one of Caesar's critics. To help Pullo, Caesar claims, would be to seem to confirm the suspicions already held by many that Pullo was acting on Caesar's orders. Octavian asks if those suspicions are correct, and Caesar assures him that he had nothing to do with Dento's death (it is revealed later in the episode, however, that Caesar did indeed order Dento's murder).
Later, Caesar informs Brutus that he wishes to appoint the younger man to the governorship of Macedonia. At first Caesar tells Brutus that he believes he is the perfect man for the job and is needed to replace the current governor, whom Caesar says is incompetent. Brutus, however, wonders why Caesar would wish to send him away from Rome, and gradually Caesar admits that he does not trust Brutus, both because of his past decision to ally with Pompey rather than Caesar and because of the recent graffiti. Brutus, feeling hurt by Caesar's lack of faith in him, storms off and renounces all friendship and loyalty to Caesar.
Caesar later discusses his future plans with Antony and Cicero, the latter of whom is pertrubed by Caesar's revelation that he plans to admit Gauls and Celts into the Senate. The meeting is interrupted by Vorenus, whom Caesar has summoned in order to discuss Vorenus' and Pullo's recent exploits in the arena that have led to the two men being worshipped as heroes by the plebs. Caesar is furious with Vorenus for disobeying his instructions not to interfere with Pullo's execution, but realises that to punish them would mean risking losing unpopularity with the masses. Therefore, he instead rewards Vorenus by making him a senator, astounding Antony, Cicero and Vorenus. Cicero vehemently protests, but Caesar remarks that he wants the Senate to be made up of "the best men in Italy, not just the richest old men in Rome." In private, Antony and Posca question his decision as it will make many enemies. However, Caesar refuses their suggestions that he double his guard, and remarks that with the great hero Lucius Vorenus by his side nobody will dare touch him.
That night, Calpurnia dreams of a flock of crows in the shape of a skull and believes it is an omen. Caesar, however, is untroubled and refuses to believe that her dream is a portent of his death. The next day, at the Senate, Brutus, Cassius, Cicero, Casca and Cimber watch with disgust as Vorenus, the Gauls and the Celts enter the Senate alongside Caesar.
On the Ides of March, Caesar arrives at the Senate house accompanied by Vorenus. However, Caesar later realises that Vorenus is no longer with him, but thinks nothing of it. He enters the Senate and Cimber approaches him under the pretext of asking Caesar to recall Cimber's brother from exile. When Caesar responds that he will address the matter in time, Cimber grabs at Caesar, who commands Cimber to let go of him. However, this is the signal for several Senators to attack Caesar, and they come at him with knives and stab him from all directions. Eventually, Caesar is left dying on the floor, bleeding profusely. Cassius tells Brutus to deal the final blow, and the young man stabs Caesar through the heart, As Caesar dies, Cassius proclaims "Thus ever for tyrants!" and Brutus collapses in shock at his actions. Antony enters the Senate, takes in the scene, and flees. Meanwhile, Servilia informs Atia and Octavian of what has happened.
Caesar's involvement in Season Two, which begins immediately after his murder, is minimal. All that is seen of him is his body as it is given last rites and finally cremated in a public funeral. Posca reveals the contents of Caesar's will, in which one-hundred denarii has been left to every Roman citizen, Octavian has been posthumously adopted as Caesar's son and heir and Posca has been given his freedom. Antony manages to dissuade Caesar's murderers from declaring their actions a lawful tyrannicide and Octavian takes the Caesar name, as it is now legally his. Throughout the season, Octavian uses this connection to Caesar to his own ends and frequently mentions his adoptive father.
- During the Gallic Wars, Caesar was Proconsul of Gallia Cisalpina (northern Italian regions of Lombardy, Venetia and Liguria), Gallia Transalpina (originally southern Gaul) and Illyria (Croatia, Montenegro and Bosnia-Herzogovina). It is thought that Caesar initially intended to wage war on the powerful Getai state in what is now Romania, but he decided to conquer the more politically-fractured Gaul instead.
- Caesar's assumed office in Rome after his war with the Optimate faction was Dictator Perpetuus (Perpetual Dictator). He is the only Roman politician to have ever held that title.
- Despite being historically documented as a rather brutal and effective warrior, Caesar is never seen once committing any killing(s) or murder(s) throughout the course of the entire series. The series does however display his various adultry activities and cutthroat tactics.