So it's not surprising that when the Killzone sequels were announced that people approached the news with more than a little trepidation (and let’s not even touch on the web-based blitzkrieg caused by the infamous PlayStation3 trailer). After all, who wants to go through a vicious sack-beating more than once? Frankly life's just too short.

Where Killzone: Liberation for the PSP makes a good initial impression is that it is clearly not a first-person shooter. Now the viewpoint is distinctly a top-down, isometric view reminiscent of classic titles such as Crusader and Little Big Adventure and despite having no control over the camera this works well and allows you to see more of the battlefield. The radical visual change catches you off guard a little and leads you to believe that Guerrilla have actually put some thought into the game and credit where credit is due, they have instead of just trying to shoehorn more of the same into a portable system.

Liberation is set two months after the first game with the Helghast invasion force stuck on planet Vekta following their failed attempt to take over the ISA's orbital defence platform. Battered but certainly not broken, the Helghan army shows no sign of giving up, leaving Captain Jann Templar with no time for R&R for his role in protecting the platform. Instead he is tasked with rooting out and destroying the remaining forces.

This takes place over a series of four main missions, each of which is split into smaller chunks where the player primarily controls Jann to complete vital objectives such as rescuing and escorting hostages or simply eradicating all Helghast forces from an area. At key points control is also given over his squad, but unlike the first game where direct control of teammates could be swapped at will, here they can only be controlled indirectly and given orders to attack and defend.

Combat is satisfying, with Jann able to use a range of ISA and Helghast weapons such as automated pistols, assault rifles and rocket launchers, with most of these being familiar to players of the original. Weapons can be scavenged from downed enemies and crates, or unlocked permanently by using cases of money that are hidden around the levels. Since the game is not first-person anymore, the inclusion of weapons such as the sniper rifle might initially seem a bit redundant but is saved by each weapon having noticeably different attributes in terms of range and capability.

Jann doesn't have infinitely deep pockets either so only two weapons can be carried at any time. Similarly grenades are also available but limited to being able to carry two different types at once, although thanks to the ragdoll physics it is extremely gratifying to lob them over objects and scenery to obliterate the Helghast. Ammo is plentiful but rather than just having to pump enemies full of bullets and hope for the best, they (and some items of scenery) have an energy bar that gives an indication of how much more damage they can take. Aided by the new viewpoint the player can also utilise the environment as cover with all this combining to give Liberation a loose real-time tactical slant, topped off by nifty slow motion action and sound effects when accessing the command menus during the heat of the battle.

While all this adds something different, it's where the game's control issues become apparent. Moving in and out of cover feels very stop-start. The huge scope of the controls will lead to moments of confusion and frustration as the player tries to take cover, command allies and return fire, which stunts the tactical aspects that would have otherwise worked well. The game is certainly no pushover with the Helghast coming in various shapes and sizes such as standard infantry units, grenadiers, attack dogs and even tank divisions, all of which are forthright when it comes to attacking and flanking. Although this ultimately leaves little time to perform the fingerwork needed to do everything at once and leading to more frustration.

At timesthe player is also given the opportunity to control vehicles like tanks and while this undoubtedly adds variety to the onfoot sections, it gives a separate restrictive control scheme to get to grips with. The player can spend considerable time familiarising themselves with these layouts (and indeed they will have to as there is no way to customise the controls), but it seems at odds with the heavy 'pick up and have a brief play' philosophy that has been applied to the level creation and mission structure. It all gives the impression that Guerrilla struggled to cram in every aspect they wanted, regardless of the limits offered by the PSP's button layout.

A checkpoint system is used to ensure that players don't have to backtrack too far when they die, but the ghosts of Killzone's past are not fully exorcised and the level design can cause problems. Various parts of the levels branch off and can be explored to find things like money cases, however doing so can trigger checkpoints that overwrite the previous ones. These checkpoints can only be activated once and the system works fine until you go off to explore and trigger a checkpoint that sends you someway back if you die. It's clear that the exploration idea was done to try and prevent a sense of linearity and add some extra depth but it will be little consolation as, teeth-grindingly furious, you are forced to travel back over large portions of the level again and again.

For those that can stick with the game or who adored the first, there is plenty to keep them occupied. The main game itself is challenging, if rather short, but it also includes mini-games that are opened after completing each chapter of the story. These include target practice, defending objectives or catching spider mines within a time limit and scoring highly unlocks additional skills such as being able to carry more grenades or ammo. Rather than feeling tacked on these skills are extremely useful during the main campaign and are worth the effort of trying to obtain them.

There is also a co-operative mode built into the game allowing two players to tackle certain missions alongside the Ad-hoc multiplayer modes. Guerrilla has also promised additional downloadable content, although at the moment nothing has been added.

The good news is that despite its faults, Killzone: Liberation is far superior to the original PlayStation 2 game. The bad news, and perhaps of more concern, is that it is still far from being a standout title. While more thought may have gone into the game, it still ends up feeling distinctly average in terms of design and implementation. This leads to the bigger question of whether or not Guerrilla are actually capable of producing a decent game, or is it once again a case of them being overambitious with the platform they are working on? They say that third time is a charm and with that in mind the ultimate acid test may be what happens when the game's hotly anticipated sequel lands on the PlayStation 3.