The Spartan begins Rome, after defeating the Minotaur in the sewers.
The player is then notified that Roman soldiers have cornered Electra, and the player must kill the Roman soldiers. If the player is too slow in this task, the soldiers will kill her and you will have to start all over again. The enemy is not that hard to defeat; it only consists of a Centurion and some Roman soldiers. After defeating them, Electra tells you they have captured Castor and will execute him in two minutes.
Get to Castor as fast as you can, and ignore the Praetorian soldiers that come to stop you. Go up the stairs and kill Castor's tormentors, which are two Centurions. Once done with that task, you and Castor will fight together, wiping out the soldiers. As you get near one of the Colosseum's entrances, Gladiators will break out and fight. You need to kill them all before proceeding.
After defeating all the enemies, you will set up some explosives. Unfortunately, the explosion results in a pile of rubble, which separates you and Castor. Castor will go back for Electra while he tells you to go and find Tiberus. As you enter one of the Colosseum's arena entrances, Ares interrupts you and you have a short and brief flashback. On the arena, Gladiators are fighting the Romans. Join the fray, and make sure you kill everyone in the arena to proceed. There are six shrines in the arena; keep these for the fight later on, and do not use unless necessary.
After killing everyone in the arena, Praetorian soldiers come in from the two smaller entrances. Once all of the soldiers are vanquished, Assassins begin to drop from the ruined ledges. After defeating the Assassins, regular Archers and two Infernus show up guarding another entrance. Kill one of the Infernus, who will then run screaming and blow up the door allowing you to enter.
After that, keep proceeding and defeat all the troops until you reach what seems to be a booth. There are corpses everywhere, and at the end is a locked gate with Assassins in it. You can't attack them (unless you use arrows), and you decide of another way to get through. Go down from the gate and you'll see a group of soldiers protecting some explosives. Kill them, and heal up your health and restore your mana for the incoming fight. Light the bomb up and a cutscene occurs, which the bomb crumbles the support, which causes the floor above to collapse and kills all the Assassins. Go up and you'll encounter Tiberius. A cutscene ensues, in which a fearful Tiberius jumps to his death rather than be killed by The Spartan.
Down in the arena, Ares is waiting for you. Once done with the cutscene, you need to kill Ares. There are corpses all over the place (including Infernus and Spartan sapper corpses). Ares has several moves of his own, including mirror images. These mirror images are capable of dealing damage, and he makes 2 copies. The copies can be killed in one hit. Ares is capable of reviving the dead, and he will do it to six random corpses on the field, so be careful if you execute one that's an Infernus or sapper as it results in damage on you. Ares final attack is a flame which heads for you. If you lure it on him it stuns him briefly and deals damage onto him.
After killing him, a final cutscene ensues.