The Animus is gaming's greatest get-out clause. Invisible walls, erratic NPC behaviour, narrative inconsistencies and technical snafus can all be explained away by the fact that you're merely experiencing a (mostly) sophisticated virtual reality simulation. If guards are attacking you despite your notoriety being zero, well, that's just a bit of dodgy Animus programming. And if Niccoló Polo is refusing to follow you because he's busy hovering above a rock? Glitch in the Animus. Pedestrians merging into Ezio during a dialogue sequence? Animus.

Yet it can't excuse everything. Assassin's Creed Revelations has been developed by six studios across three continents - and in a little over twelve months. It shows. Revelations' fractured campaign doesn't suffer from a lack of ideas, but the new inclusions either fail to add anything meaningful, or, in some cases, actively detract from the experience. You can't really blame Ubisoft for not trying here; there are plenty of new additions to deflect the common accusations hurled at yearly updates. You can, however, criticise the way in which these concepts have been integrated.

Revelations is, of course, the third game starring Ezio Auditore da Firenze, and while the official line will no doubt claim otherwise, it's clear it wasn't ever meant to be this way. But Ezio's popularity demanded his story be expanded, even if the main thread of the plot is now in a holding pattern, circling slowly, waiting for the go-ahead to descend.

So, with Ezio's tale still to be resolved, it means Desmond, the guy we're supposed to care about, spends the entire game unconscious, his mind having taken him to a blue-green world of floating monoliths and glowing lights. With a beach and a body of water gently lapping at the shore, this peaceful idyll is not a bad place for a coma destination - at least until it's interrupted by someone whose identity I probably shouldn't reveal. Either way, it's merely a conduit for Desmond to once again don the simulated pauldrons and virtual vambraces of our ageing Italian stallion, who's now in Turkey and fighting Templars and Byzantines, presumably because the developers fancied a change of scenery.

From the outset, you notice something's different about Ezio, and I'm not just talking about his craggier-than-a-cliff-face character model and grizzly beard. There was always flamboyance about his kills as a younger man; while the series has never shied away from the brutality of the creed's methods, here was a man who seemed to enjoy his work. There are still flourishes here - at times Ubi seems to forget that Ezio is well into his fifties - but there's a cold savagery to the violence at times. It's instantly obvious in the opening battle, when our hero jabs his sword upwards through a guard's skull, then spins the head around horribly. Gorehounds will delight in the nastier kills, from throat-slashing to slow motion eviscerations, but it's not a change that sat particularly well with me.

Nor is it the only curious change. Ezio might be out of Italy, and with it his usual variety of roles as assassin, mentor, landlord, and recruitment officer now feel like a burden. In former games these elements felt optional, repaying your time investment by making life easier, either by affording you regular back-up in difficult encounters, or swelling your bank balance to make the most powerful weapons and armour affordable. In Revelations your tasks feel more like busywork, as purchasing buildings automatically increases your notoriety. To reduce the threat of constant street violence, you'll need to bribe heralds or kill Templar messengers, or try to capture a Templar den. When you do so, you'll need to position a Level 10 Master Assassin to make your ownership permanent, otherwise the Templars will contest the den you took from them. At which point Assassin's Creed goes all tower-defence on us.

That's right: Revelations genuinely does feature a tower defence mini-game. It's viewed from a pseudo-third-person perspective as you stand on a roof, sending archers and riflemen onto other buildings and barricading the streets to prevent invaders reaching your den. The viewpoint isn't particularly helpful, as it only gives you a view of a small section of the street, while unit assignment is unintuitive and fiddly. While after the first instance these sequences are essentially optional, you'll be forced into repeated bribes, den captures or assassin missions to avoid them. And until you've got a group of Master Assassins, if you want to expand your property empire around a certain area you're going to have to bite the bullet. Should the Templars take the den, you'll have to repeat the capture process, a process made more annoying by the cowardice of every den leader, as they'll sprint to safety as soon as you're spotted. Given that you need to use Eagle Vision to identify them - at which point you have no way of telling whether other guards have spotted you - this often leads to moments where you're surrounded by six enemies and your quarry has often escaped, forcing you to wait a day before attempting another capture.

Combat is much the same as it was before, although you'll find yourself running into tougher, more heavily-armoured enemies this time. Fortunately, bombs tip the balance back in your favour. Crafting them should be fun - you need to pick the shell, gunpowder type, and the effect they'll have - and you have a range of options at your disposal, from lamb's blood that showers enemies with the red stuff, causing them to briefly panic, to phosphorus light shows designed to distract. Pyrite coin explosions send pedestrians scrambling for fake money (handy when you're trying to create a diversion) and, of course, there are the more destructive kind including - joy of joys - sticky bombs. Trouble is, there's little reason to use anything other than impact bombs. Lob a fuse bomb and you'll need to wait three seconds for the explosion. Similarly, tripwire explosives are a nice idea in theory but take a good few seconds to set up, and it's all too easy to get caught laying them on the route of an enemy patrol.

The bombs also feel a little out of place. In a game where several missions require you to avoid detection - or reward you with full synchronicity for not being spotted - it's odd to be encouraged to lob bombs everywhere, even if a tactically placed smoke bomb can be a simple way to pass by a well-guarded door. Yet it can be something of a lottery whether they'll catch you through the fog. More than once I strolled off nonchalantly while guards were coughing and frantically wafting the air, only to see that telltale red triangle that showed I'd been discovered.

Brotherhood's tombs, meanwhile, have been replaced by linear platforming sections featuring lots of collapsing scenery and last-minute escapes where Ezio clings on for dear life. All we need is the occasional shout of "oh, merda" and we'd have a 16th century Uncharted. Still, as much as these moments owe a debt to Naughty Dog, they're undeniably exciting, presented with a cinematic flair that easily matches their inspiration.

Yet such sequences only serve to highlight the lack of inspiration in the game's other missions. The midsection of the game settles into a strange routine, as you run a series of basic errands (one has you tail a florist then pick tulips), before perching on top of a building and moving a cursor round to highlight glowing markers in order to uncover a secret location. The end justifies the means, but compare Revelations to the previous games' winning mix of scripted sequences and open-ended stealth-action and it comes up wanting. Even at its best, there's nothing here to match the atmospheric delight of Brotherhood's beautiful basilica in Palazzo Laterano.

Nowhere is this lack of inspiration more obvious than in Altair's missions. The five MacGuffins Ezio seeks during his Turkish adventure each corresponds to a playable section with the original game's hero. Yet the tasks are often tedious. In the first mission, you simply run up a hill and kill lots of guards. The second asks you to climb a building while bursts of energy from the Apple of Eden conspire to knock you down. The final memory involves little more than walking from one place to another.

Yet if the missions are left wanting in terms of structure, mechanics and fun, they're woven into a well-written narrative. Where Uncharted 3 sacrificed a cohesive plot at the altar of explosive action, Ubisoft is happy to let the story drive the content. That it's not an entirely disastrous decision is mainly thanks to the quality of the new characters. Ottoman Assassin Yusuf, your guide for the early game, and a recurring presence throughout, is a likeable addition to the cast; the feisty and intelligent Sofia, who thaws a little of Ezio's icy demeanour as the game progresses likewise. That the bad guys don't quite have the same impact is an understandable problem for an assassin: if you're doing a good job, your enemies don't tend to last very long.

Moreover, it ties all three narrative strands together more efficiently than before. Altair and Ezio's connection in particular is explored more thoroughly. There's also a satisfying finality to the story of one of these characters, though the send-off the other receives still feels slightly incomplete.

And what of Desmond? Well, he gets a few brief moments to shine, but it's a new optional element - unlocked from collecting Animus fragments spread across Constantinople - that sees Nolan North earn his fee. In Desmond's Journey, we learn a little about his past, as our hero reminisces of his life before the Assassins intervened. Bizarrely, this is achieved by exploring abstract environments in first-person while placing platforms under your feet. At first, it seems relatively simple, but then you'll have to get past moving lasers that disintegrate the platforms you place and a strange, dark substance that moves anything floating above it. These sequences are either some of the most irritating spatial puzzles you've ever played or the least efficient storytelling device ever. Either way, as you sit through the lengthy credit crawl you'll wonder how, of the several hundred people involved in the game's development, no one suggested they might be a terrible idea.

A disappointing campaign is supplemented by an updated version of one of the most unfairly underplayed and underappreciated online games in a long time, an inventive cat-and-mouse variation on multiplayer standards that deserved more attention than it got. For its early weeks at least, Brotherhood's online component was a rare treat, until a number of players seemed to catch on that running around on roofs was the secret to multiplayer success and interest waned. The new game does little to rectify that, in truth, with a few perks to boost speed that only seemed to encourage rooftop sprints in the games I played. The contested kill idea, meanwhile, seems unbalanced, apparently rewarding button-mashing after the fact to drop the murderer's points. There's an attempt to provide a narrative grounding for the multiplayer, with some guff about Abstergo your reward for levelling up, but like many of the solo ideas, it seems a little half-hearted. Ubisoft is promising tweaks and patches, but while with the right set of players it's still one of the most entertaining multiplayer games around, it's unlikely to win anyone over from Call of Duty or Battlefield for anything more than a week-long fling, if that.

So, for the first time, a new Assassin's Creed game is worse than its predecessor, the first time the short development period has had a noticeable impact on the game's quality. It's a game of nearlies and might-have-beens: summed up by the hookblade, a supposedly key new feature which in practice merely extends Ezio's reach slightly, and allows him to glide down the occasional zipline. Its setting, while evocatively realised, is less interesting than before, and its systems and feedback loops just aren't as satisfying as they used to be. As much as I've enjoyed spending three games with one of gaming's most charismatic leads, perhaps it is time he - and Ubisoft - moved on.

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