From Wikipedia
Killzone: Liberation is a third-person action game for the PlayStation Portable, developed by Guerrilla Games, and is the sequel to the 2004 first-person shooter Killzone. Killzone: Liberation was released for on October 31, 2006 in the US and on November 3, 2006 in Europe. On March 5, 2009, it became available for download from the PlayStation Store. The game chronologically continues two months after where the first game ended. Killzone: Liberation should not be confused with Killzone 2 for the PlayStation 3, the next addition to the Killzone series. Killzone: Liberation is presented as a third-person shooter game, where its home console counterparts are presented as a first-person shooter. It is currently the only PSP installment in the series.
Gameplay
This game features a top-down isometric view in contrast to the original Killzone game for the PS2, which was a first-person shooter. The resultant game can be seen as a mixture of a shooter and a dungeon crawler.
The player can only control Jan Templar, although he can board and control several vehicles: heavy machine gun turrets, a tank, a hovercraft and a jetpack. During some levels a 'buddy' can be ordered around (although some are unarmed). These 'buddies' are normally teammates (Rico and Luger) or people that have to be rescued (The three VIP's in the third mission, first chapter and Evelyn in the third mission, fourth chapter).
By collecting various pickups in the form of money cases, the player unlocks new weapons to use in the game. These weapons can be found in various weapons caches in certain levels. After enough money is amassed, the V2 upgraded weapons become available. The weapons in the caches are also converted into the V2 upgraded versions.The unlockable weapons are: The ISA Assault Rifle, Helghast Assault Rifle, a Helghast machine pistol, a Helghast sniper rifle, an ISA shotgun, a Helghast revolver, an ISA crossbow and a Helghast chaingun (The ISA assault rifle is the default weapon). After downloading chapter five updates for the ISA flamethrower and Helghast rocket launcher are available to unlock.
The player can also unlock upgrades to Templar's abilities by completing challenge games within certain limits (Such as shooting X number of targets without shooting the 'civilian' targets). These are ranked in bronze, silver and gold. Upgrades come in the form of carrying more items, hitting harder in close quarters combat, performing actions faster, unlimited ammunition or upgraded health.
Multiplayer
Players can play via Ad-Hoc. Players can choose a team (between ISA and Helghast) and also choose a character in the multiplayer menu. There are a variety of game modes, including Deathmatch (where player kill one another regardless of team), Team Deathmatch(where teams kill one another) , Deathmatch Duel (where a slightly smaller portion of the map is cut off by barbed wire so players have a smaller area to manouevre in) , Capture the Flag (where teams try to get the enemy flag and bring it back to their flag) and Assault ( Where the Helghast team has to plant 3 charges of C4 around an ISA beacon and the ISA have to try and stop them). (Note: Deathmatch Duel is available after chapter five download). Infrastructure was only made available for download, and is not initially available for "out of the box play." The game features headset compatibility for online play.
Downloadable upgrade
The game was originally intended to have downloadable content from Day 1, but due to technical difficulties, the Downloads promised were initially un-delivered. On May 30th, 2007 the free downloadable upgrade was released. The upgrade included two additions:
Online multi-player support including two new multi-player maps and a new multi-player game mode.
Chapter 5 of the single player campaign which concludes the game storyline.
Also, the upgrade is delivered in two parts. You have to visit the download page of the game twice.