Apart from the Science Fiction story and the ridiculous overpower of the protagonist, how correct is this game historically?

Other than the setting - not massively.

The main targets all died on that year under unknown circunstances. The cities' layouts were based on historical blueprints from that same year (although they can't be that accurate due to size). There are many more, but don't count on assassins with wrist blades and dressed in white patrolling the Holy Land... They did exist, but not like that.