A significant improvement over the original, but held back by uncomfortable controls and a number of flawed mechanics.
As has just been mentioned, the original had you swapping between the four characters as you proceeded through the storyline, and while this allowed for differences in ability to shine through, in the end it results in a lack on sympathy with the lead cast - you feel for the Master Chief as he does his thing and you can get into his shoes, but when you aren't playing as one character for any more than a couple of levels at a time, it's difficult to feel the same way towards Killzone's guys.
The original game was a disappointment, so this resulted in a blissful lack of hype, hype, hype for the PSP-exclusive sequel, KIllzone: Liberation. No longer was it expected to beat Halo. No longer was it even expected to be anything special.
No longer, in fact, was it even expected to be a first-person shooter. While Killzone 1 half-heartedly shotgunned its way into the ranks of generic FPSes, Liberation invents a new genre and makes it its own. You control just one character - Captain Jan Templar - for the duration of the game, and instead of seeing the grim world of KZ through a camera planted firmly in between Templar's eyes, you view him in first person in a 2.5D isometric perspective.
This immediately eliminates the confusing layouts and lack of peripheral vision that plagued KZ1. Brilliant. Your enemies, the armoured, vicious, cunning mutant Helghast, are no longer bumbling Cockney buffoons who stand in front of you, lazily grunt "Frag out!" and toss a grenade with all the enthusiasm of a house brick, but each enemy now represents a genuine threat that must be systematically eliminated. They're still Cockney, though.
Unfortunately all this "systematic elimination" amounts to in reality is the two of you crouched behind opposite sides of an item box, you ducking when he's firing at you and shooting back when he isn't. When the game becomes harder in later levels and throws more enemies at you, this system really comes into its own, however. Bullets travel at about the same speed you can on one of two levels - standing up, or crouching down. What this means is that if you are skilled enough, you can run right up to a Helghast machine gun emplacement ducking under its bullets and then melee the operator to death. Speaking of melee, it knocks weaker enemies to the ground for you to unload a clip into them, but after the second chapter it does not knock them down, resulting in "melee battles" that are essentially you and a Helghast shock trooper amusingly slapping each other until one of you falls down dead. And it is usually going to be you, because you are very likely going to be very low on health, ammo and everything else very frequently in the game.
Item boxes are among Liberation's biggest shortcomings but also the basis for a lot of the game's strategic elements. As Jan Templar, you can carry one weapon, then a choice of explosives - you can either carry two frag grenades, two smoke grenades or one remote-detonated mine - and finally up to three satchels of C4, though you will never carry more than one at a time. Periodically spaced throughout the game are Helghast item boxes which contain a variety of such gear - and they are the only places in the game where you can change weapons or pick up grenades. And even then, only if the weapon you want to change to is in that particular item box. And once it's gone, it's gone. And to add insult to injury, when accessing one, the game does not pause but merely slows down - forcing you to watch a hail of bullets searing towards you in slow-motion as you scramble for your weapons and health.
You will also find yourself frantically jogging (there is no sprint command) across half the level to reach a box to pick up a health refill - as there is no regenerating health mechanic either. You can occasionally find health packs in crates that will refill your health by 25 or the full 100 points - though invariably you will find a +100 pack when you are at near-full health anyway, and only a meager +25 when you are in desperate need of some. However, this mechanic is more likely to force you to think than it is to frustrate you enough to throw your PSP to the ground and stamp on it.
No, what's going to make you do that is the atrocious "evade" mechanic. At one point in the game you are up against a Helghast colonel with a huge walking tank, the primary weapons of which are a pair of surface-to-air mortar launchers. However, being a Helghast, he doesn't like you very much and has decided to fire them at you. The only way to avoid damage is to roll out of the projectiles' way, and the button command to roll is to double-tap the L button. On the original (1000-series) PSP at least, being forced to do that repeatedly puts your fingers in a very uncomfortable position and could even cause you to slip on the controls and satisfying the blood lust of a certain cocky Cockney colonel.
The voice-acting is on the whole very well done, with the Helghast in particular being given a range of stereotypical Nazi-like phrases and the aforementioned colonel constantly shouting equally stereotypical "Die, die, DIE!" phrases as you break your hands trying to jump out of the way of his mortars.
Speaking of bosses, there are only two in the game, but they are at the end of difficult, self-contained levels - and there is no function to save and quit halfway through a mission - if you leave for whatever reason, you're playing that level again. Making these bosses standalone levels would have probably been a more sensible move.
Killzone: Liberation is a very well-made game that stands tall as a massive improvement over the original , albeit with a bevy of minor but frustrating flaws. I've said just above that the colonel spews out death threats - one of which is my personal favourite: "Can you outrun a bullet, Vektan?" It's just a horrendous piece of irony that in Killzone: Liberation, you actually can.