During the medieval period, how common was the use of assassins and assassination?

by ColonelGaraffi

I'm interested in the medieval period, and whether assassinations were really as common as the media presents them. Also, how would one have gone about hiring an assassin, and what methods would they have used? Would some targets, such as clergymen, have been considered above assassination? Sorry for the many questions, I've just become really interested in this topic (mostly after playing medieval II total war).

yosemitesquint