Given this original entry, I don't know how the Ass Creed series ever even got off the ground. This game violates nearly every rule of good design, good gameplay and consideration for the end user while doing little more than looking really nice.
It's sort of a stealth game, most comparable to Thief I suppose, except instead of stealing stuff, the central goal is to Hashashinate people. Like the first Thief, however, it often gets away from the central focus and has you doing a lot of out-and-out combat instead, and also gets bogged down in cumbersome "supernatural" story and a lot of cutscenes. It's also kind of like Prince Of Persia For Dummies; you perform all sorts of jumping, running and acrobatic moves, but it's all done automatically for you at context points so long as you are holding down the Run and Super Parkour buttons once you get there.
The story is actually set in the present day; you're a bartender named Desmond who apparently used to be part of some nomadic assassin tribe, but quit to live a normal life as a regular jerk in the city. Unfortunately, the evil Shady Cyberpunk Megacorp finds out about Desmond, kidnaps him, and puts him in a machine called the Animus which forces him to relive "ancestral memories" encoded in his DNA. In Desmond's case, this is the memory of assassin ancestors during the Crusades era, who are basically the Hashashin working against the Knights Templar. The Templar are trying to find some mysterious artifacts that will let them rule the world with supernatural powers and etc., in typical Templar style. For some reason Desmond can't just tap directly into whatever ancestral memories Shady Megacorp is after, he has to actually relive a bunch of parkour action assassination missions, and if he deviates too far from the memory (i.e. gets killed), he drops out of it and has to start over.
That's already a pretty contrived and unwieldy setup right out of the gate; it doesn't get any better as it goes on and aliens (I shit you not) get involved. It also has the effect of distancing you from a potentially very interesting environment, these lovingly detailed recreations of these 12th century cities. Outside of guards and targets, all the interaction you ever have with people is to push them out of the way, or get begged at by them (and then push them out of the way.) The game also claims "open world" immersiveness, but you're constantly walled off along very linear corridors by "memory limiters" (literal invisible walls that appear if you try to go off the path), and during the occasions that you are set free to roam about a city, there's really nothing to do there other than admire the scenery. The environments are pretty, empty Hollywood set pieces that feel like Potemkin villages to move around in.
You'd think context-based action would be the simplest type to implement and plan around, but somehow it turns into a total shitshow here, mostly when you're trying to climb walls (which is a regular activity.) It's far too easy to accidentaly make dude kickflip himself off a wall or heave himself bodily to the left or right when simply trying to reach over to an adjacent grip point, and go plunging off a cliff to his death. When even a system as oversimplified as this gets this bad to wrestle with, you know your design has gone really wrong.
The game is also far too padded. Nearly every mission has you walking to or from your cliff-based Hashashin Fortress HQ, along the route of which absolutely nothing at all happens. Well, except that a thousand beggars nag at you, and you have to shove them aside. Then there's the droning, uninteresting cutscenes and dialogue sequences. Then there's the weird attemps to be "unexpected" and "immersive" like the bizarre quasi-tutorial the game opens with (where you appear to be trapped in the Matrix), but just fall flat and leave you wishing the game would just get the hell on with a fun bit that you can actually control.
The game started off with a great concept (parkour Hashasin organization fighting Templars in a really nice-looking historical recreation) and then proceeds to totally tank it by making bad decisions in almost every other aspect. Setting the whole thing in Aperture Science in the future was, IMO, a terrible move as compared to simply writing a story set in the actual time period, and making the world feel more alive. Then you introduce all the convoluted alien and magical power shit and it gets even worse. At least a third of the gameplay time is boring padding in between droning poorly-edited conversations and just walking around through a pretty but otherwise uninteresting environment, and at its worst moments, the game seems to be trying to out-Kojima Metal Gear Solid 2. Apparently later entries in the series did this concept much better; you can safely skip the first one.
* Gameplay Video