Siege of Viana

In 1503 Rodrigo Borgia, Grand Master of the Knights Templar died of poison courtesy of his son Cesare Borgia, and with his death Rome was fully liberated from the Borgia rule. Soon after the death of his father, Cesare Borgia, commander of the Papal Army, was arrested by the new Pope, Julius II. In 1504 Cesare was exiled to Spain and imprisoned at the Castle of La Mota, Medina del Campo, from which he escaped and joined his brother-in-law King John III of Navarre.

The Battlefield
After Cesare's arrest and exile to Spain, Ezio Auditore - who had remained in Rome - was conversing the possibility of Cesare still being a threat with his closest friend Leonardo da Vinci at the Assassin stronghold of Tiber Island. Ezio believed that Cesare remained a threat even after his arrest, and worried about Cesare's previous claims that chains would not hold him and his vow not to die by the hands of man. Leonardo then encouraged Ezio to use the Apple to see into the future.

Ezio proceeded to do so despite his own initial protest and, upon gleaning information from the Piece of Eden, immediately left Rome for Viana, Spain. As Ezio makes his decision to leave, Leonardo questions Ezio about the Assassin's guild, to which Ezio replies that it was rebuild to last with or without him. With his farewell, Ezio left Leonardo a large amount money to help with his miniscule salary, guaranteeing him that the two would see each other again.

By 1507 when Ezio finally tracked Cesare down to Viana, the siege had already begun. As the soldiers fought each other on the battlefield, and cannons destroyed the nearby castle, town and countryside, Ezio wasted no time in charging into the fray. Ezio soon located Cesare alongside his men on the battlefield, and without hesitation, he lunged towards Cesare with his Hidden Blade at the ready. However, Cesare was able to hold off the Assassin's attack, demanding to know how Ezio discovered him. Ezio quickly claimed the Apple that Cesare stole from Mario Auditore led him here.

Despite Cesare holding of Ezio's killing blow Ezio eventually gained the upperhand and overpowered Cesare, striking him across the throat. However, Cesare's armor protected him, and he fled, commanding for his soldiers to spill Ezio's blood. As he fended off the reinforcing soldiers and attempted to pursue Cesare, Ezio was then stunned by a wave of cannon balls that impacted the battlefield all around him. After regaining consciousness amidst several corpses of enemy soldiers, Ezio then pursued his nemesis.

Riding through the Viana warzone on horseback attempting to reach what was once an olive field. Cannonballs eventually killed his horse, forcing him to travel by foot. As Ezio drew closer to the besieged town, he eventually arrived a Viana soldier's territory. After stealthily killing off the patroling soldiers and dispatching any who challenged him Ezio subsequently arrived at the besieged town. Ezio then evidently noticed Cesare, accompanied by several guards, approaching the town. A woman rushed to Cesare, seeking help in looking for her injured son, but she was promptly killed and discarded by his men, with Cesare continuing onwards to the city fortress.

The Viana Castle
Following some distance behind, Ezio fought his way through the town, killing off the attacking Navarre and Viana soldiers. Subsequently reaching a burning seige tower, which he attempted to climb in order to progress to the other side of the town. However, as he neared the top, the tower had sustained heavy damage from cannon fire and collapsed, forced Ezio to perform a Leap of Faith and search for an alternative route to the Castle walls.

Battling his way through the other section of the town Ezio discover another siege tower at the Castel walls, which before proceeding noticed Cessare battling above. He successfully climbed it and made his way across the battlements, where he finally confronted Cesare. Cesare faced him head on, taunting him into battle. Cesare prompted Ezio at first by using his firearm, but Ezio managed to fend it off. Cesare then summoned guards to combat Ezio as the battle progressed.

Final confrontation
As the siege raged all around them, the two men battled for the first and last time. Cesare attacked Ezio with speed and skill, aided by his sword and pistol, while also periodically calling in waves of Navarre soldiers to aid him. Despite this, Ezio was able to defeat all of the soldiers and overpower Cesare by breaking off pieces of his armor with the Hidden Blade. He eventually pinned Cesare down and told him that a true leader empowers the people he leads. Cesare, enraged and knowing his death was inevitable, repeatedly shouted that no mortal man could kill him, thus Ezio then declared he would leave Cesare in the "hands of fate" and threw him off the castle wall to his death.

Aftermath
After Cesare's death, Ezio returned to Rome and reunited with the other Assassins. Ultimately deciding that the Apple was too dangerous for the hands of man, Ezio would later seal the Apple away in a Vault beneath the Santa Maria Aracoeli.

Trivia

 * In the story trailer for Assassin's Creed: Brotherhood, Ezio and Cesare are seen confronting each other alone on the castle wall. However, this scene never happened in the actual game.
 * Although the Siege of Viana is where Cesare Borgia died in actual history, there is very little recorded information on the event. It is something of a historical mystery in terms of the outcome and important specific details, such as the amount of troops involved.
 * When starting the game Desmond will attempt to access Sequence 9, and when the player ends up in the battle, Ezio tells Cesare, "--Mario Auditore led me here!" When accessing Sequence 9 after completing the previous 8, Ezio speaks the missing first part of his sentence, "The Apple you stole from Mario Auditore led me here!" This is because Sequence 9 was corrupted at the time that Desmond first attempted to access it, but was fully functional afterwards.
 * The first time Desmond attempts access Sequence 9, Ezio will shove a Viana soldier aside as he charges towards Cesare, however, when Sequence 9 becomes fully operational, he charges straight passed each soldier until he confronts Cesare.
 * When the player reaches the castle walls a wave of Navarre soldiers will run straight past Ezio to the siege tower, these soldiers most likely climbed the siege tower and later provided aid to Cesare during his battle with Ezio.
 * When replaying the memory, after Ezio releases Cesare off from the castle walls, his body can be seen on the ground when looking down from the battlements right before the memory fades out.