Exit from the Animus.
Talk to Lucy.
Use your bed.
The next morning, you'll talk to Vidic again.
Use the Animus.
The people in the town will be under Al Mualim's control.
Fight your way up to the castle.
Other assassins will help you defeat some enemies.
Walk through the crowd of people at the castle entrance.
Climb the first set of stairs and go through the open door there.
The gate will shut behind you.
Al Mualim will use his powers to freeze you in place.
He will summon the men you killed. Defeat them, they aren't very tough.
Al Maulim will then split into many forms. Only one will be the real enemy.
Use your Eagle Vision to reveal the true Al Maulim (in gold) and attack him.
There will be another cut scene with Al Maulim.
You will begin another fight with him. When you hit him, he will vanish.
You will lose all your life. Run around the area until you find Al Maulim again. Be careful, because he can kill you in one hit right now.
Once you attack him, you'll gain your life back.
Keep attacking him. He'll vanish like before each time you hit him.
After a few times of vanishing, he won't do that anymore. You'll just fight it out until one of you dies.
After several hits, you'll get a cut scene of his death.
The treasure will reveal the locations of the remaining copies of it.
You'll awake from the Animus. You're no longer needed, so they plan on killing you, but Lucy helps save you.
You can use your Eagle Vision to see hidden messages all over the room.
Use the Eagle Vision on the wall above your bed.
The credits will roll.
After the credits are done, you can access the computers if you took Lucy and Vidic's pens.
This will let you read emails, revealing some information on what was happening.
Congratulations on winning the game! You can get back in the Animus and go to any point of the game that you'd like to play.