The Assassin's Creed series follows Desmond Miles who, despite being a mere bartender, is the descendant of a series of skilled assassins and is forced by an evil corporation to re-experience past memories held within his DNA by the 'Animus'. The reason for this are the 'Pieces of Eden' that are powerful relics that can help the Knights Templar sculpt the future of humanity. Luckily, Desmond's experiences in the Animus start giving him the skills and abilities of his forebears so in later incarnations he becomes a little more personally useful and a little less of a mere conduit for his far more skilled and interested predecessors.
The game itself is a third-person stealth-based athletic game where you have to climb, jump, and backstab your way through the various missions.
One vital aspect of Assassin's Creed is that every structure that you need to climb has grab points conveniently located on it. The benevolent architecture is loaded with platforms, protruding beams, and flagpoles.
While in a roleplaying game you don't require official 'grab points', the Storyteller always needs to allow for the possibility so that nearly any building they choose is potentially usable. Whether it's a drainpipe that can conveniently hold their weight or bricks with gaps sized for finger tips, where there is a will there must be a way.
Assassinations also need to be possible. If you can't one-hit-kill an unwary individual, why would you bother trying? Even in World of Darkness where the characters only have 7 hit points it can be difficult as you would still need to roll 7 successes on the dice. In any game where you need to wear down the enemy's hit points, it's often better to either charge en masse or pepper them from afar rather than bother sneaking up to them and striking them from behind. While a D&D or Pathfinder Rogue can do a sneak attack, their damage output is often not high enough to kill an enemy outright, they can sneak attack from afar or while flanking rather than having to sneak up behind them, and your players might want to play something other than a rogue.
There are several options to integrate stealth kills into the game. You might make it an automatic kill if they strike an unsuspecting target with the right weapon. You could simply give them bonus damage dice, or better yet, bonus automatic damage if they strike unseen.
In my new Wodterday Campaign (the Demon: the Fallen dystopia one I'm creating), I'm going to make them succeed on three rolls for an automatic kill against any human opponent. If they're going with the stealth option, then they must successfully roll a Stealth check, a Medicine check (to ensure they hit the right place), and a Weaponry check. Or if they're going for an athletic option such as dropping down on them from above, they must successfully roll a Stealth check (to catch the enemy unawares), an Athletics check (the momentum from the acrobatics means they don't need to be so precise), and a Weaponry check. They only need one success on each and certain equipment bonuses and specialties can give them better odds.
In the videogame, the characters' le parkour is generally automatically successful. Sure, you can jump in the wrong direction or swing out too far, but it's generally pretty intuitive and easy.
On the other hand, in a roleplaying game there are normally a great risk for loss. On a d20 system, the players will roll a 1 at least 5% of the time. If they have a 10 in Acrobatics, they will fail a regular DC of 20 half of the time. In BRP, their chances of failure may be even higher. In World of Darkness, they could roll no successes on 10 dice. When you have multiple players all making the same attempt, the chances for catastrophe grow.
Of course, you could simply have it that failure gives the enemy a Perception check to spot you versus your own Stealth check. Or you could have it that failure simply slows you down so you don't finish the move and instead hang suspended from the flagpole. Or you could find some way to make it a rote action unless its a dramatically important moment (often a good idea unless your players like making a lot of rolls). On the same note, you don't want to make it too automatic too often, or else there'll be no threat and they might as well just be walking along the ground. One middle ground would be to have them roll per stretch of buildings they're climbing along and have failures provide unusual situations they have to solve. That way the players can even look forward to a failure so that they can think their way out of the corner they've painted themselves into.
Stupid guards are an important component of any game with stealth elements and is no less important here. Sure, they may remain wary after briefly spotting the player characters, but if they acted with true realism, one wrong move would make things nigh impossible for the characters to continue stealthily as a single wary guard alerts all of the other guards and they remain watchful for hours. Or perhaps even call in reinforcement. While it's often straining the immersion to breaking point if you have the guards do a quick search and then call it a day after the third suspicious noise, it's often a good idea to simply have the guards err on the side of laziness. After all, they may have worked that shift for years and nothing has happened so far.
Other options to deal with guards include those same ones used in the game. Include places to hide (perhaps make luck rolls to see if the guards happen to search them), crowds to lurk within (so long as their armaments are less obvious than Ezio's), and you could also just stab the guard in the throat before he can scream for help, which is a pretty realistic answer to stopping a guard from screaming.
When Ezio kills a target, the world becomes a white void room around them and they can carry on a quiet conversation while the target slowly dies ... even if the circumstances of the assassination would give them no such privacy. Since the only way to get such a conversation happening when the player characters are going to stealth kill someone is to go with something like this, you could feel free to use the same tactic and create some sort of occult reason for it such as the psychic link between murderor and victim. If you want to keep some level of realism, you could only have the target talk to them and slowly die in situations where the assassins aren't in a currently threatening situation.
Everyone needs to be on the same page with this style of game. If one person lacks Athletics and/or Stealth (or your system's version of it) then they will either force the rest of the party to avoid the stealthy and athletic options that are this game's bread and butter - or the party will be forced to leave that person behind. Where possible, don't give the players blank character sheets. Jot down the minimum requirements in Athletics or Stealth so players don't forget the importance of them.
A campaign based around Assassin's Creed, or including elements of it, would certainly appeal to Tacticians as you have to think about the best way to approach a situation, especially as you're working in a group. Action Heroes with an interest in stealth will enjoy the varied action sequences, from assassinations and ambushes to horse riding and duels. The ample history and interesting citiscapes may well keep Explorers reasonably happy with it as well, especially if you describe the world of roof tops and wall running to give them an experience they may never have in real life.
Investigators will need a double cross or other mystery to spice it up and may get bored unless you give them a task, like keeping their allies and enemies straight. Communicators will enjoy meeting new people but may try and slow the game down quite a bit to get full enjoyment out of it. The odd impersonation, dodgy meeting beneath a bridge, and masked ball mission might re-kindle their interest if it dips.
It might be worth opening a dialogue with everyone so that these two groups understand that you'll give them their moments (though it won't be a focus) and so that the other three styles of players understand that the game needs to slow down on occasion so these two styles get their chance to shine. After all, there's nothing that breeds resentment in players more than quietly putting up with something out of common courtesy only to have another player shatter your chance to shine out of a mistaken belief that it was an irrelevant moment that no one else needed.
If you'd like to take a look at the trailer to learn more about it, you can check it out here. If you'd like to read the sort of tropes that Assassin's Creed used, you can find them here.
For the next Game Translation, you have a choice of these: Left for Dead, Fall Out 3, The Sims 3, Half Life 2, Prototype, Skyrim, Deus Ex, The Last Express, Mass Effect, Realms of the Haunting, and pretty much any survival horror or horror game. If no one picks anything by next week, it'll be Fall Out 3.
By the way, Dreamers and Dicepools also looks at how one might create mechanics for a game from the ground up at allow folks to play Assassin's Creed.
If you want to see the list of games I've done thus far, you can find the Game Translation series starter over here.