I played this game a while ago. It's awesome. Now, there's one tip you missed to give out.
Assassinating a rival is not necessarily dishonourable! When you infiltrate his house and reach his bedroom, you'll find him asleep on his bed. Now you have two options. The dishonourable, cowardly way is to close enough and kill him with your sword (the same way you kill the guards). This way you get rid of him easily, but lose honour.
Now, the honourable way is to get closer and closer to him (if I recall correctly) until he wakes up. Then you'll have to fight a duel with him, and if you manage to kill him you won't lose any honour and he'll be out of the way. That's the right but harder way to do it.
Hope that helps.
k: