After completing the game's single-player campaign and digging into its multiplayer offerings, I can confidently say that I still have no idea. It could accurately be categorized as either, or even both, but ultimately that distinction's not all that important. Any way you slice it, Assassin's Creed: Brotherhood is an absolutely exceptional game.
The expansequel (?) starts off with a very literal bang, taking place immediately -- in both of the franchise's timelines -- after the conclusion of Assassin's Creed 2. Ezio e società return to their estate in Monteriggioni, having successfully perforated the army of conspirators responsible for the pruning of the Auditore family tree. Unfortunately, their peace is short-lived, as the Borgia clan (one of the ends left loose in AC2) makes a visit to their well-tended home to -- without going too far into spoiler territory -- return the favor.
Once again, Ezio's actions are driven by revenge, but serving up that cold, cold dish isn't quite as easy as finding a target and stabbing it this time around. The Borgia have completely overrun the territory of Roma, controlling its people with a totalitarian fist and a dozen-odd tall, highly flammable towers. In order to obtain vengeance, you'll first need to cut the legs out from under your enemies, toppling their control over the citizens of Roma and severing the support of their like-minded allies.
It's a different storytelling approach from Assassin's Creed 2's decades-long revenge epic and, admittedly, it lacks some of the last game's narrative punch. Though it's pretty satisfying to take your enemies' lives apart brick by brick, the tasks you're given while accomplishing this brutal goal are scattershot, occasionally you entirely unsure of what you're supposed to be doing, or why you're doing it.
The campaign is a bit shorter this time, too, though the quests therein are a great deal more dynamic than they have been in previous Creeds. More than a handful of story missions are on-rails, cinematic sequences, typically involving horses (which play a much larger role in Brotherhood). Other missions see you using your usual sneaky tactics, but in more scripted scenarios; for example, one chapter has you infiltrating a passion play in the middle of the Colosseum, which is not only exciting, but also -- possibly -- boundlessly controversial.
While the story's hit-or-miss, the number of sidequests available to Ezio are just mind-boggling. There's the usual object hunt for flags, treasures, feathers, viewpoints and secret messages from the still-cabalistic Subject 16. There are platforming-centric "Lairs of Romulus," basically the Assassin's Tombs from AC2. There are Borgia towers to find and destroy, allowing you to renovate businesses and tunnel entrances around Roma and giving you quicker access to important parts of the city.
You'll also have a more active role in the upkeep of the four guilds: Courtesans, Mercenaries, Thieves and Assassins. The first three can be strengthened by purchasing empty lots for them to settle in, doing missions or completing Guild Quests -- a type of leveling system which you scale by performing a certain number of stealth or combat maneuvers, and that grant you discounts when hiring these groups in the future.
While not sandwiched between the Animus' ergonomically designed armrests, Desmond and company's hunt for the Pieces of Eden reaches a decidedly compelling zenith. Believe it or not, more meaningful character development takes place in the "real world" than it does in 16th century. By the end of the game, you'll be absolutely slavering for an Assassin's Creed in which Desmond never steps foot into the remembered lives of his ancestors.
Not only does it constitute something entirely new for the franchise, it represents a cerebral antipode to almost every other current-gen game's multiplayer offerings.
Of course, Assassin's Creed: Brotherhood's multiplayer component doesn't really have much of a place in the "sequel vs. expansion" argument -- it's clearly neither. Not only does it constitute something entirely new for the franchise, it represents a cerebral antipode to almost every other current-gen game's multiplayer offerings.
On the surface, the game's four online modes are fairly single-faceted, though that one facet is simply brilliant. You're basically just playing hide-and-go-seek. Players are represented by one of a handful of characters, then dropped into a small area populated by hundreds of doppelgangers. Players have contracts to assassinate other players, whom they must pick out of the crowd of duplicates, while avoiding conspicuous actions which may alert their prey (or their own predators).
You can probably understand my confusion when trying to figure out exactly what Brotherhood is. If you didn't play the last game, you'll have absolutely no idea what's going on during the single-player campaign. At the same time, there's so much new content here -- the multiplayer component in particular absolutely needs to be played by anyone with a current-gen console and a high-speed internet connection.
Ultimately, it doesn't matter whether Assassin's Creed: Brotherhood is a sequel or expansion. I'm not sure even Ubisoft knew which it was while developing it. All that matters is that if you let that ambiguity keep you from playing it, you'll be doing yourself an awfully huge disservice.
This review is based on a near-finished debug build of Assassin's Creed: Brotherhood for the Xbox 360, provided by Ubisoft.