Here's a little thought experiment for you. Forget Killzone for a second. Imagine a new PSP game being announced with the following features:

- 16-level single player campaign

- multiple upgradable weapon types

- a (simple) cover system

- vehicles

- squad command system

- competitive online multiplayer (8 players)

- local ad-hoc WiFi co-operative mode (6 players!)

- free DLC accessible in-game which adds another chapter (four levels) to single-player as well as some more online modes and weapons

- separate Challenge modes which provide RPG-lite upgrades to your character

... Not bad, right? This is one of the most developed PSP games ever made. But is it fun? Yes, it's pretty fun. It's also balls-out hard.

You play as ISA commando Jan Templar from the first Killzone game, in a series of counteroffensive missions occurring two months after the initial Helghast invasion of Vekta. (Also making an appearance is Rico Velasquez, who is a key character in Killzone 2). The idea here is that Scolar Visari – the Helghast Emperor – has charged his top commander (named Metrac) with holding southern Vekta after the first offensive. Of course, you need to go mess with his shit.

Playing Liberation is an interesting experience because of the tactical nature that the viewpoint offers. There is no camera rotation whatsoever; what you see is what you get, even though it's all rendered in 3D (and with Havoc physics, no less). Thankfully, when standing behind an obstructing wall you can still see an overlay of your character, and these moments are rare due to the careful level design. The left shoulder button locks you into a strafe, while the right shoulder button activates your crouch, which is a key element. Lots of cover is provided in the environment, and the flow of the game is generally a stop-and-pop affair, with you rushing to advance to the next bit of cover while Helghast troops try to flank you or flush you out with grenades. There are also lots of traps to deal with, such as laser tripwires, landmines, and spidermines. The latter offer another tactical wrinkle: spidermines look like regular mines but "wake up" and come crawling after the nearest biped when disturbed. I say "biped" because the AI on a spidermine is pretty dumb – you can easily use them to your advantage by putting a bullet in one when a Helghast is standing closer to it than you are, thus causing the irate munition to attack them instead. Other defences like the laser tripwires can be disabled, although you must pause and hold the X button for a number of seconds to do this, which of course makes you a sitting duck. As you make your way through each level you are given clear objectives as voiceover from a mission commander. A full map is always available under the Select button.

Along the way you encounter all manner of toys to play with. There are of course the standard assault rifles, shotguns, pistols, sniper rifles and missile launchers to pick up. You can only carry one weapon at a time, however various "weapons cache" crates scattered around each level offer the chance to swap in your old gun for a new one, as well as picking up health restores and different kinds of grenades or explosives. Later in the game you can encounter more exotic things like a futuristic crossbow, remote-detonating mines, and flamethrowers (with the DLC). Also present are two vehicle types: a tank and a hovercraft. The controls are thankfully fairly simple, and these things basically provide a sort of armoured suit for Templar, as well as two kinds of weapons instead of one. You can also get a jetpack at one point. Jetpacks, as we all know, are automatically awesome, so I need not extoll the virtues of this decision.

There is also a command system for working with an AI partner (Rico). Tapping up on the d-pad puts the game into a slow-motion mode where you can tell your partner AI character to take a specific cover point, plant explosives, or directly attack specific Helghast enemies, cycling through choices with d-pad left and right. It's a straightforward, effective way of adding another tactical layer to the firefights, and Rico can truly save your ass when you've got three or four Higs firing at you... given the right guidance.

The Challenge stages, another single-player mode apart from the Campaign, provide a series of specifically controlled levels that have a time or score limit assigned to a particular scenario. For instance you might run through a level full of target dummies, avoiding shooting "civilian" green pop-ups while gunning down red "enemy" ones, within a set time limit. Another scenario has you planting C4 in a circular arena while fending off Higs from every direction. These challenges award bronze, silver and gold trophies, which in turn accumulate to give the player unlock points for special abilities, such as carrying more grenades or disarming traps twice as fast. You can choose where you spend these upgrade points once you have enough to meet the "price". You can also collect briefcases hidden in crates on each level, giving you Vektan dollars. These can be spent on user-selectable weapon upgrades.

As I mentioned earlier, Liberation is a hard game. Expect to die a lot as you get used to the controls and figure out what's what. For instance: you have very little chance of surviving a spidermine the first time you see it. Thankfully the game is old-school not just in its difficulty curve but in its design: enemies pretty much spawn from (obvious) doors in exactly the same numbers each time, although what they do after arriving can vary quite a bit. You learn to look for the more dangerous Helghast troops such as those armed with missile launchers and shotguns by their character designs. The famous Helghast glowing eyes also play a role here, turning from yellow to amber to red, depending on their state of alarm or awareness. Checkpoints are fairly generous as well, but this can lead to some awkward and frustrating respawns, as it's not uncommon to hit a checkpoint while in the middle of a firefight... and yes, you will start again in the middle of that exact same fight, with the health and ammo you had when you dinged the (invisible) checkpoint. It is entirely possible to screw yourself by crossing the right magic spot in a weakened condition, and your only recourse is to restart the entire level. Melee combat can be very tricky to get used to as well, as it is contextual. A "fist" icon will appear above the player when close to a crate or an enemy; fact is, the Helghast hit really hard and fast if you get close, and it's not uncommon to have a heavy trooper simply beat the shit out of you should you get within arms length, no matter how much you mash that X button. Levels are quite long as well, probably too long for a portable game, to be honest – I found that the average level took me anywhere from ten to twenty minutes to play. (I pretty much only play the game at home, for this reason).

As you can see, Killzone: Liberation has an extraordinary amount of depth and sophistication for a PSP game. I haven't even touched on the multiplayer aspects, which look amazing: an example mode is Assault, where one team has to plant three C4 charges around an ISA beacon while the other defends. This game supports six players in co-op mode locally as well, so if you can somehow wrangle six people with PSPs, that could be a real treat (the game will share one copy over WiFi for these purposes; you don't need six copies of the game itself). While those following Killzone 2's development point to things like Call of Duty 4's influence for ideas like the RPG-lite elements, it seems that many of these aspects were already well-developed in Liberation.

If you are interested in catching up with some of Killzone's backstory, and own a PSP, this is a great buy for $20. Between the choice of playing the original Killzone on PS2 or Liberation, I cannot help but recommend the latter strongly over the former. Killzone 2 plays differently enough from it's original ancestor that you would gain no real advantage, while the storytelling, music, combat, and overall polish of Liberation set it apart. Killzone 1 was an ok game, too ambitious for it's own good and marred by technical glitches. Liberation is on another level altogether.

Killzone Liberation is available now as part of Sony's Greatest Hits lineup for the PlayStation Portable, with an MSRP of $19.99.