The only other thing to add to merchants is I think you get a bonus when engaging another merchant if you own the territory.
I find merchants particularly good in the early game - as the Moors I have them down on three of the Timbuktu resources and they bring in about 30% of my income. This reduces over time as the trade and tax income grows, but when you're constantly at war its useful.
Assassins are frustrating but can be good if you micro manage them and for those that say they don't bother with either I'd just add that as they are part of the game, shouldn't we be playing them?! After all, the objective here can't be simply to win can it?