In a perfect world, all video games would be reviewed on a level and unbiased playing field - but unfortunately, this isn't the case. Tenchu: Shadow Assassins is a game featuring lots and lots of ninjas, and as a result there's an unwritten law that dictates that it must be absolutely brilliant. Ninjas are one of those rare commodities that maintain their coolness regardless of whatever is happening elsewhere within popular culture; it's a terrible crime to make a lacklustre ninja game, a sin that can only be punished by a SHURIKEN TO THE FACE... or in this case, by a slightly lukewarm review.

There was once a time when the Tenchu series was something of a powerhouse. Along with Metal Gear Solid, the original PSone Tenchu was responsible for pioneering the art of third-person stealthing. Unfortunately the franchise has since fallen upon hard times, and while Shadow Assassins is far from being a complete disaster, it's equally far from being a triumphant return to past form. Gamers who've long followed the exploits of Rikimaru and co will find much to enjoy, but newcomers may struggle to see past the poor AI, inconsistent design and frequently frustrating gameplay.

At heart, Shadow Assassins is largely the same old Tenchu game we've been playing for all these years. You take control either the silver-haired Rikimaru or ninja nymphet Ayame, then head out on a series of stealthy assassinations in the service of your clan lord. Direct attacks are a distinct no-no, so instead you creep your way around levels and murder the guards when their backs are turned. The series' trademark violent executions are correct and present, and this time you'll have to carry them out by meeting the game's demands for specific waves and thrusts of the Wii remote. This time around, they're given the label hissatsu, and they're as violent as ever. The motion controls fare reasonably well in this department, although they're somewhat unfair on beginners: Once you've learned which movements are used for each context you'll be fine, but expect to fail a couple in the early stages thanks to the relatively harsh time limits for each QTE. It doesn't help that some executions give you a choice of movements to follow while others expect you to do everything indicated on screen - and at first it's hard to tell which situation you're in. With time, you'll learn where you have choice and where you don't, but this is still a problem that should have been avoided through better explanation.

One fairly significant change to the established Tenchu formula is the fact that missions no longer take place in large, open arenas. Instead each level is now broken up into several small stages with a clear goal marker that must be reached in order to progress to the next section. While this bite-sized arrangement suits players aiming to speed-run the game (there are ranks and rewards for swift and thorough play) it also has the effect of making everything seem a lot more confined. Past entries in the series offered the player a multitude of ways for handling a given situation, but here you often feel like you're being herded towards the "correct" path of action. This feeling is further compounded by the fact that you can no longer use a grappling hook to go climbing over walls and rooftops - in fact, your ninja assassin feels like a bit of a gimp: he can leap up onto the inner rafters of buildings and clamber along raised ledges, but only when the game prompts you to do so. The rest of the time, Rikimaru and Ayame find their paths blocked by piles of scenery items - a distinctly old-school slice of lazy level design that does the game no favours.

Giant Ninja likes to eat unwary diners.

There are other limitations too. You can now only carry three special items at a time, and several of the more interesting weapons from previous games are conspicuous by their absence. Poisoned rice balls are nowhere to be found, but you do get access to a helpful ninja cat - a strange addition that almost addresses the balance. Less amusing but more useful is the water pipe you use to extinguish flaming torches from afar. Sticking to the shadows is the rule of the day in this game. A rather snazzy-looking moon icon in the bottom-left corner of the screen indicates how hidden you are: if it's bright and clear, people can see you; if clouds have swathed it in darkness, you're largely invisible. To aid you with your furtive movements, you now have access to a sort of ninjitsu sixth sense that highlights enemies and their lines of sight, as well as useful items and paths hidden from view.

Any time you're around hostile forces you'll obviously want to remain hidden from view, skulking in the dark or by using some form of scenery - often a bush. Since on-foot movement is a bit clunky and imprecise - and since From Software have declined to give us a manual crouch button - your best bet for stealthy progression is to use a hayate - a slick roll into the nearest cover, conduct by flicking remote in the relevant direction. This isn't nearly as bad as it sounds, but like most of the other Wii-centric controls, there's an overbearing feeling that things would work better on a standard joypad (i've not yet had a chance to play the PSP version of this game, but it would be interesting to see how it compares).

While the hayate and the aforementioned hissatsu work relatively well, there's another set of movements that come across as bottom-rate Wii gimmickery. If you get spotted by a guard and fail to roll away into cover, you'll be challenged to a fight. Normally you'll just disappear in a puff of smoke and reappear at the start of the level (more on this in a bit), but if you're unlucky enough to be carrying a ninjato sword... well, may the immortal gods have mercy on your soul. In this case, you'll have to deal with an excruciatingly bad first-person QTE where you try to block incoming sword attacks by aligning your remote with an on-screen indicator. Succeeding here requires a Nostradamus-like ability to see into the future, the reflexes of Spider-Man and the patience of someone who doesn't want to kill themselves while playing pointlessly poor mini-games.

That shuriken looks like it's made of chocolate. Perhaps that's why now they bounce off people's faces.

As a result of these horrible fights, you'll probably avoid carrying the ninjato pick-up wherever possible. This in turn means that you'll be sent back to the start of the area each and every time you're spotted by the enemy. There's no re-spawning of dead guards when you get sent back in this manner, so provided that you manage to take out at least one man on each cocked-up attempt, you'll eventually get through to the next zone. This setup was presumably designed to help less-skilled players, but it's far from ideal. There's something ridiculous about the way a ronin will spot you, scare you off and then shout "Yeah, you better run!" before resuming his post - only to die 30 seconds later after you've sprinted through the empty rooms you've already cleared.

It's also fair to say that Shadow Assassins is fairly irregular in terms of obeying its own logic. Fine, so dark areas will hide you - but you'll only be able to hide where the game decides there's enough shadow, and that's far from obvious without using the special ninja sense. Bad guys will spot you crawling about on the other side of the room, but they somehow won't notice if you jump into the pond they're standing next to. Shuriken, the lethal throwing stars that got Barry Hodkins in trouble for buying when you were on your French exchange, are no longer dangerous: throw them at a guard, and he'll just be a bit annoyed. At one point during my play test, I attracted a guard by flinging a star at his head, then climbed up onto an overhead beam. Normally, I'd be able to grab him and strangle him as he passed underneath, but on this occasion I couldn't - because your enemies are apparently invincible when they're annoyed.

Despite all these gripes, Shadow Assassins is actually a half-decent game. It looks rather good, with some nice lighting effects and attention to period detail, and it's backed up by an impressive orchestral soundtrack. Sure, it's frustrating as hell a lot of the time - but Tenchu players are used to this, to the extent that it's almost part of the formula. The overbearing failing with this game is that it simply fails to innovate. When there are products like Metal Gear Solid 4 and the Hitman series out there on the marketplace, albeit on more powerful platforms, it really doesn't do to start cutting down on a player's freedom. There are two campaigns to play through, an extra series of 50 brief assignments to complete, and a smattering of new items and artworks to unlock; to this extent, the game is quite generous, but I think that most players would have preferred less content and more room to manoeuvre.