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This is one of the best Assassin's Creed yarns yet: a dark tale laced with mystery, political drama and shock revelations. It gives the old assassin some unexpected depth, and answers many lingering questions. Only a constant feeling of familiarity blights the experience.

The year is 1511, and Ezio's travels have brought him to the Middle East. He's in Constantinople - better known today as Istanbul - searching for five keys hidden there by his ancestor, the legendary assassin Altaïr Ibn-La'Ahad. The keys unlock a secret buried beneath Masyaf, an ancient castle that was once the headquarters of the Assassin Order, and that fans will recognise as the starting area of the first game. The bad news is that the Templars, sworn enemies of the Assassins, are looking for the keys too.

Known historically as the crossroads of the world, Constantinople is the 16th Century equivalent of New York City: a bustling metropolis teeming with people of all races, religions and cultures.

The first time you clamber to the top of a large tower and see the hazy afternoon sun falling over the buildings, which seem to stretch for miles, you'll spin the camera around in awe. It's one of the most beautifully immersive open world settings we've ever explored, rich with history and character.

Each area of the city has its own distinct feel. The Galata district, home to the Assassins, is made up of busy, winding streets lined with beggars and merchants. The Imperial district is a relative paradise, with green, flower-filled gardens and palatial mosques. The docks are dominated by huge galleons, their colourful sails reflected in the shimmering water. It really does transport you to another time and place.

It's massive too, which is where Ezio's new Hookblade comes in handy. This acts as both your hidden blade - the trademark weapon of the Assassins - and a new way of navigating the city's rooftops.

As you leap towards a building, hold Circle and Ezio will stretch his arm out and slam the hook into it with a satisfying, tactile thunk. This allows you to cross wider gaps, and 'double jump' while climbing to scale buildings faster. It's only a slight tweak to the series' established platforming mechanics, but a useful one nonetheless.

It's not all about Ezio. Desmond plays a significant role in Revelations too. In the real world he's comatose, but his mind is active and trapped inside the Animus on a bizarre desert island made up of abstract shapes and flickering data streams. Here he meets the enigmatic Subject 16, who tells him that the only way he can free himself is to complete as many of his ancestor's memories as possible.

It gets weirder. Whenever Ezio locates five data fragments hidden around Constantinople, optional side missions called 'journeys' are unlocked on the island. These are perhaps the strangest new addition to the game, like a cross between Portal and LittleBigPlanet.

They're viewed from the first person, and set in chambers that wouldn't look out of place in an Aperture Science facility. The goal is to get from one end to the other by making bridges from 3D rectangles and wedges. It's simple at first, but they get more complicated when gravity reverses, or changes direction, moving your shapes around.

This makes it sound more impressive than it actually is. It's conceptually interesting, but the puzzles are poorly designed and often frustrating. It desperately wants to be Portal, but it has none of the ingenuity.

The main motivation for completing these journeys is revealing Desmond's past. He's always been fairly one-dimensional, but this gives him some presence as a character. It's just a shame Nolan North plays him so closely to Nathan Drake. The similarity is distracting.

But back to Ezio. Of the many new skills he picks up in Constantinople, bomb making is the most entertaining. Using ingredients looted from chests or bought from stores, you can craft three different types of bomb: lethal, tactical and diversion. The type of casing you use affects the bomb's behaviour. You can make them bounce off walls, stick to surfaces or explode instantly on impact.

Caltrop bombs cover the ground in sharp spikes that can slow enemies down; useful if you're being chased. Throw a blood bomb at an enemy and they'll panic, thinking they've been fatally wounded. Datura bombs release a cloud of poison gas, and tripwire bombs can be used to set traps.

Those are just a few examples. There are potentially hundreds of combinations. It's an enjoyable toolkit, and in a way reminds us of Arkham City's gadgets. You can perch quietly in the rafters, mixing different bomb types to confuse enemies, temporarily blind them, or separate a lone soldier from a group for a silent assassination.

Most stealth sections can be completed by simply alerting the guards and stabbing them all to death with your sword, but if you want to play more strategically, the bomb system offers a wealth of tactical options.

Oh, and it's funny too. There were a few laughs in Assassin's Creed II, but they were relegated mainly to cut-scenes. In Revelations, some missions are designed purely to make you laugh. One of the best sees Ezio dressing up as a wandering minstrel, complete with a feather in his cap and leg-hugging white tights. As you prance around a party in the Sultan's palace distracting the guests with your off-key singing, your fellow assassins slip into the group and quietly kill key targets.

It does remain a rare moment of imagination and humour in an otherwise overtly serious game. But when you're making Ezio sing and twang his lute, or firing poison darts at guards from a distance to make the locals think they've fallen under a gypsy curse, you can't help but admire the game's humour and creativity.

Other attempts to add variety aren't quite as successful. Two on-rails horse-and-cart battles are clumsy and irritating; something that would have been far more entertaining as a cut-scene than a twitchy, barely-interactive mini-game.

Occasionally, as you move through crowded areas, a Templar assassin will stab you in the back, severely reducing your health and forcing you to give chase - even if you're in the middle of a mission. In fact, many of the missions are unremarkable and meandering, making the story about five hours longer than it needs to be.

However, the hunt for the Masyaf keys brings out the best in the developers. Each one presents a different challenge - be it sneaking through a cavernous cave network undetected, or chasing Templars through crumbling ruins as they hurl bombs at you.

These self-contained moments are some of the game's best, and each one offers a new type of gameplay. They're often heavily scripted, reminiscent of Uncharted, and somewhat brief, but you'll always look forward to the next one blinking on your mini-map. Then when you finally reach the key itself, things get even more interesting.

It's no secret that the third playable character in Revelations is Altaïr, Ezio and Desmond's distant ancestor, and hero of the original Assassin's Creed. Each Masyaf key contains a memory from his life that Ezio can interpret, and that you get to experience first-hand. There are five in total, and they pick up immediately after the events of the first game, spanning multiple decades until Altaïr is an old man.

These memories take place in Masyaf in its heyday, and chart his never-ending struggle to keep the mysterious Apple of Eden - an alien artefact that forms the basis of the entire Assassin's Creed plot - out of the clutches of the Templars. These sections feature their own novel gameplay twists, but saying any more than that would ruin some genuine surprises. These memories also provide closure for Altaïr's story.

This is still Ezio's game, however. You'll spend 80% of your playing time as him, and that's Revelations' biggest problem. There's a lot of very basic missions to endure between the good bits, all retreading ground extensively covered in earlier games.

The minor tweaks to the combat and climbing, and the new setting, don't cover up the fact that, mechanically, this is the same game we've played three times already. Like Brotherhood, this is less a sequel and more a very limited refinement of old ideas.

As well as two versions of Masyaf from two different time periods and the gargantuan Constantinople (and all that lies beneath it), there's a whole other city to explore, revealed later in the game. No, we didn't see it coming either - especially as it pops up ten hours into the story, just when you're convinced things are about to draw to a close. Even ignoring the optional side content, this is a big game with well over 30 missions.

And if you're invested in the Assassin's Creed storyline, you simply have to finish it - even if the idea of spending another 30 hours as Ezio puts you off. After the comparitively meandering plot of Brotherhood, Revelations' final hours not only explicitly hint at the direction of the next game in the series, but put to rest one of its biggest mysteries in memorably dramatic fashion.

It's also the end of Ezio's adventure, confirming that this is his last stint as the hero. We've complained about being sick of him in the past, but it's hard not to feel a tinge of melancholy as his story comes to an end. In Revelations, he genuinely grows as a character.

Ultimately, then, this latest Assassin's Creed game isn't a complete success. The reliance on recycled gameplay is its biggest flaw, especially if you've played the last two games all the way through. But when it's good, it's really good. There are moments that make enduring the frequent dips in quality worth it, and the sheer volume of content ensures this is excellent value for money.

But while Ubisoft have bled this chapter of the Assassin's Creed saga dry, the clues in Revelations' haunting ending suggest the next game will be something very special indeed.