Aren't we bored with Ezio by now? Assassin's Creed may be a game about exploring the past but it's high time Ubisoft turned its attention to the future – not least because we're unsure Desmond Miles can handle another cliffhanger ending. He's already in a coma.

Unfortunately, Ubisoft isn't quite ready to whisk away the curtain and show us what's going on in the real world. So this year's instalment in the Assassin's Creed series – always a jarring way to have to describe a narrative-driven video game – is stalling for more time.

Ezio's back, along with a comatose Desmond and even original hero Altair. While the name of the game may be Revelations, for this lot it might as well be called Closure.

If you do feel you've had enough of Ezio, though, remember that while he may have been the lead character in the last two games, the real stars were the cities. Florence, in particular, and last year's Rome were enormous, beautifully reconstructed visions of Renaissance Italy. The success of those games owed just as much to crawling over their intricate rooftops and delving into their depths as it did to Ezio's machinations.

Constantinople, the setting for much of Revelations, should be a fine and distinctive addition to that dynasty. Ezio journeys to the city that became Istanbul at the height of the Ottoman Empire under Suleiman the Magnificent, when it was already a crossroads between East and West where many Jews, Muslims and Christians had been forcibly relocated.

Neighbours but not necessarily friends, these different cultures invested their surroundings with a diversity that suits an openworld game. The hilly terrain should also be useful – Ubisoft estimates that 85 per cent of Constantinople is on a slope.

Video: The years show on Ezio's face, but he's still as nimble as ever.

Ezio, who is a little older and greyer but seemingly no less acrobatic, is in town on the trail of (who would have thought?) another balding, beardy Templar bad guy. On a pilgrimage to Altair's home city of Masayaf – the series' starting point – he is assailed by the Templars and discovers they are seeking five seals that are needed to unlock a library hidden beneath the city, where Altair has buried a weapon of great magnitude. Four of those seals are located in Constantinople and the fifth is in the possession of the Templars.

For the E3 demo, we join Ezio as he prepares to leave the city and seek out his Templar nemesis. First he talks to the local Assassin's guild chief Yusuf and discovers that the harbour has been blockaded with ships and barricaded with a large chain drawn from the old Tower of Gelata. (As with many things in the Assassin's universe, the chain was a real thing that Ubisoft is borrowing for its purposes.)

In good news, Yusuf has a new type of bomb for Ezio – one more than 50 times more powerful than his usual fare. Bombs will play a big part in Revelations (there will be around 300 and you'll gather items to craft them as you play). As he reaches the Tower Ezio uses one of the lighter varieties, a splinter explosive, to kill a pair of the local Janissary guards.

As he moves closer still, he gets in a fight with some more Janissaries and someone throws a smoke bomb. This clouds proceedings and gives us an excuse to see how Ezio can use Eagle Sense to pick out movements in the haze (Eagle Sense, an updated version of Eagle Vision, also lets Ezio perceive the paths enemies will take so he can set up explosive ambushes).

Once the bad guys are out of the way, Ezio plants Yusuf's bomb, steps back and then detonates it with a shot from the pistol on his gauntlet. The Tower falls in on itself and the chain across the harbour descends.

Before Ezio can make his escape, though, he still needs to deal with the blockade. So he uses a new device, the hookblade, to ride a zipline down to a nearby ship where he can take advantage of Greek Fire – the Byzantine equivalent of a flamethrower – to set the surrounding ships ablaze.

(The Assassin's game engine barely shrugs at this beautiful illumination and mutilation of so much densely packed detail, although we are seeing this demo on a high-spec PC rather than a console.)

Just as he is finishing up, cannon balls start to rain down on Ezio's position and he has to move quickly to reach his ship. Fans of the phrase "vertical slice" will be pleased by the sequence that follows, which sees Ezio run nimbly across a rope connecting two ships, jump and swing himself fluidly from an environmental trapeze, leap stylishly among burning timbers, briefly climb some rigging and assassinate a Janissary from below, cut a weighted rope and ride it to the top of a mainsail and then zipline into a double kill before tumbling elegantly onto his escape vessel, all in the space of about 60 seconds. (Requiescat in Pace, bitches.)

In the final game this kind of story mission will be complemented by plenty of side content, including more of the self-contained, almost episodic diversions that made up the Assassin's Tomb levels in ACII and the lairs of the followers of Romulus in Brotherhood. Ubisoft says these will have more of a narrative layer this time around too, rather than just being interesting and memorable curiosities.

One example of this may be the sections where Ezio relives Altair's memories – and within which you actually play as Altair. Whenever Ezio recovers one of Altair's artifacts he is transported back to a memory of the old master's life around Masayaf, allowing you to experience the assassin city at different points in its history.

Back in Ezio's present day, meanwhile, Assassin's Dens will be an evolution of the Borgia Tower, city improvement and assassin recruitment concepts from Brotherhood.

You'll eliminate Templar influence and set up a Den. It will then act as a local hub for guild activities, allowing you to add ziplines to the surrounding area and even appoint a Master Assassin who can protect it in your stead. Until then you'll be called upon to defend your Dens periodically, or at least send some recruits to do so.

And while Desmond may be in a coma, Revelations isn't just about closing the circle between Ezio and Altair; it's about bringing those two characters to a narrative intersection with Des.

Following events at the end of Brotherhood, he is now trapped in a part of the Animus called the Black Room. Apparently the White Room is the normal loading environment for the Animus – the place you see in the loading screens before Altair or Ezio's genetic memories are loaded – while the Black Room is the Animus equivalent of MS-DOS: more powerful but also dangerous, a place where you can lose yourself forever.

In a brief trailer showing some Black Room sequences, Desmond can be seen entering a doorway of blinding light on top of a grassy hill, then moving among floating white particles, geometric shapes and vast walls plastered with red writing, or staring upwards into long shafts of light and banging his fists on translucent walls. The Black Room sequences will be more puzzle than platform, and should give us more glimpses into the secrets of the Animus and Desmond's own past.

Those of us who mostly want to hear about his future, however, will have to continue waiting. This is the game that will bring the story of Ezio Auditore to an end, and deliver us to what the developers are referring to as a "nexus point" for Ezio and the series' other protagonists – a sort of narrative crescendo that leaves us good and ready for Assassin's Creed III.

Whether that wait will prove too long for some remains to be seen, but if Brotherhood taught us anything, it's that you should never write off a series that's always so capable of surprising.