Battle of Arsuf
The battle of Arsuf took place during 1911, the date of the Third Crusade. During the Second Crusade, the Crusaders, under King Richard, were able to repulse an attack by the Saracens at Acre. This let them retain a strong foot hold on ground that was strategically important as the Acre was the only port available to the Crusader army.
Eventually, King Richard led a counter attack himself, and let the city of Acre in the hands of his three most trusted men. He led the march just south of Acre, attempting to flank the Saracens.
Arrival of Robert De Sable
Robert, seeing that he was the last target, let Maria impersonate him so that he could gain time to reach Richard in Arsuf. Altair only knew of Robert's deception when he attended the funeral and was attacked by Maria, whom he defeated in battle and spared her life. Maria, hearing Altair's questions, informs him arrogantly that Robert has gone after Richard, convincing the King to join alliances against the Assassin Order.
When Altair hears this, he goes back to the bureau to inform Malik of the impending threat on Masyaf. He tells Malik to find what he can as he rides for Arsuf.
The Battle
The Saracens have apparently set an ambush against the Crusaders(They did not have negotiations yet, as Robert is still conferring with Richard) as they have archers shooting at the advancing Crusader Army. Several Crusader strongholds have been overrun by the Saracens.
When Altair reaches the outskirts of Arsuf, he battles his way through numerous Saracen soldiers, who have set ambushes against the assassin. Altair, however, mows them down as he continues towards the Crusader Army. As he continues, he finds himself fighting crusaders as well. They share the same fate as the Saracens.
The duel with Robert De Sable
Altair finally reaches Richard, with Robert at his side. He is stopped by Templars and questioned by King Richard. When he explains his story, Robert talks back, saying that they should not trust an assassin. Richard, however, seems confused, and instead lets them fight to see who God sides with.
Altair faces Robert's Templars first. IT was a tough battle, but Altair killed all the knights nevertheless. Robert then finally confronts Altair himself. Robert attacks with speed and power, but Altair was more skilled, as Robert loses his life to the assassin. Robert explains his part of the story, and shocks Altair. He then has a small conversation with King Richard before finally leaving for Masyf to do one last thing.