First off, this is in no way related to any feature that it is in the works. It's just something about which I was personally curious.
After having read some of the threads about assassination missions for the game, I have been trying to find out how pre-modern assassinations were actually carried out.
Basically, as far as I am aware, two scenarios that often come to mind when gamers think of assassination -- an assassin sneaks close to a target and dispatches him or her using a missile weapon, or an assassin breaks into a protected area, kills the target, and escapes -- were very, very rare, if not completely unheard-of.
Rather, the archetypal pre-modern assassination of a public figure was something like the killing of Julius Caesar, in that...
1) The victim knew and to some degree trusted the killers
2) The killers used knives
3) The killing took place in the course of a coup d'etat, which meant that the assassins did not need to escape
Other modes of pre-modern assassination include:
* Overpowering the target by weight of numbers. This can include murders by a bodyguard (multiple Roman emperors), collaring the target in public with a large group (the Pazzi plot in Florence or the murder of Byzantine emperor Leo V)
* Going up to the target in broad daylight, stabbing them, and either not planning to get away or relying on confusion to get away (Hashashiyin, the Sicorii). After the 16th century you start getting pistols as well (William the Silent), but keep in mind that these are also carried out at very close range. A considerable proportion of these assassinations are religiously motivated -- indicating that escape is very much a secondary priority
* Poison (usually best when administered by the victim's wife or servants)
Even today, it's very hard to assassinate someone from long-range. Virtually every assassination of which I can think uses a firearm fired from close range (no more than 10-20m), explosives, or a melee weapon (be it knife or poison-tipped umbrella). Even modern states which regularly sponsor assassinations and who could presumably train a cadre of marksmen don't normally use them to off their opponents. However, there is very well-known long-range assassination -- JFK -- which tends to heavily shape the popular perception of assassinations.
Even with a high-powered scoped rifle, it's hard to predict a target's movements, find a spot which has a good line of sight but is secluded enough to set up with a rifle and wait, get a weapon there without being spotted, and hit a moving target surrounded by guards, retinue, petitioners, etc. It would be considerably harder with a crossbow, which are (1) more difficult to conceal, (2) less lethal, (3) have much less range, (4) are more vulnerable to wind, adrenaline-induced hand shaking, etc, (5) cannot be reloaded in a timely fashion.
There is one case in which an assassin tried to use an arquebus from concealment, had a plan of escape, and did actually wound the victim -- the attempted assassination of Admiral Coligny in Paris in 1572. Note however that the assassin probably had the backing of some very highly-placed courtiers, if not the king himself,
As for sneaking into someone's bedchamber, I'm guessing that it also didn't happen very much, if at all. Medieval palaces were just too crowded. This assumes that the story involving Salaheddin al-Ayyubi and the note left by his bedside is apocryphal.
All of this means that there is very little chance that a professional assassin who is targeting a high-profile target would be able to do the deed and collect the paycheck. This leaves the field to the Hashashiyin and others who have other-worldly motivations.
What about ninjas, you ask? I am much less familiar with the Far East than I am with the Middle East and Europe, but this article suggests that while there may have been ninjas, of a sort, they weren't much use as assassins. It's not a scholarly piece, but it does seem to make a serious effort to review contemporary sources. http://www.illuminatedlantern.com/cinema/archives/ninja.php
Note that I am sure you had medieval contract killers -- they exist in pretty much every contemporary anarchic situation, so you would expect the same laws of supply and demand to function in the Middle Ages as well -- but I doubt they spent much time going after well-protected high-profile targets, and I doubt they were particularly well-trained. Rather, a lord might hire a couple of thugs to ambush a creditor, a troublesome spouse, an unruly tenant, etc.
Any thoughts? Anyone have any counter-examples?