How do you unlock arsuf on assassins creed?

Answer 1

You can only get to it after Altair fails the third and final assassination in Jerusalem. If you exit Jerusalem and go into the Kingdom and check the map, you'll see that the way to Arsuf has opened up. It's Southeast of Acre.

Answer 2

if i remember correctly u have to assassinate all 9 and Robert de sable and then u unlock arsuf