I've mildly enjoyed the Killzone games throughout the years: the PS2 original didn't impress me at all, but Killzone 2 and 3 were solid FPS games that had just enough personality to make it feel memorable, particularly in its heavy gameplay feel. Shadow Fall was pretty average in comparison, sadly. But before I played any of these games, there was Killzone Liberation, which is my entry to the series. I enjoyed it back then on the PSP, but never got to finish it. After finishing the PS5 port, I can say that it actually holds up quite well.
The most obvious difference in the gameplay from the mainline titles is the isometric view. It plays like a twinstick shooter, except it's single stick, since the PSP only had one. Moving and aiming are one and the same, but you can lock to a target you're currently aiming so you can strafe around said target. You can also lock on objects in the environment, like an exploding barrel, but only when you're crouching. Crouching is also used for hiding behind cover, and you can roll forward to dodge some projectiles if needed. With all of these mechanics in mind, its a heavy, relatively slow paced shooter that is more about executing a plan you thought of in the moment rather than just fast reflexes. The most flawed part of this is that the targeting system can be a bit flimsy and awkward sometimes.
The gunplay experience depends on the weapon you have, like being forced to get close to enemies if you use a shotgun. There's good enough enemy variety to make you want to change weapons before/during the gunfights, which I thought was a fun way of pushing the player to be more adaptive. You can only change weapons at specific weapon caches throughout the level. You also don't have regenerating health so you have to find medkits. Even when you have a lot of ammo and grenades, its not that easy to properly defend yourself, so the game always keeps a decent level of challenge.
There's a solid selection of weapons available, and more are introduced as you go. All the weapons offer their own style of approach, and that's cool, but there's just certain weapons that are obviously better in most scenarios (looking at you, Helghast Chain Gun). I particularly take issue with the sniper rifle, which is not very fun to use because of how slow it is. But hey, it's real fun to blow up enemies with explosive weapons and see them fly around in classic late 2000s era ragdoll fashion.
You can pick which weapon to start a level with, but you need to unlock enough collectibles in the levels to do so. There's also unlockable extra perks if you do well enough in the challenge missions. I'll say that these side objectives are worth doing for the rewards. Unloading my 8 bullet hard-hitting upgraded revolver with unlimited ammo makes me real happy.
I think the biggest weakness of this game (other than the boring story) is the variety of the missions. The level design are usually decent, but there's a few missions early on that backtracks through previous levels, and when that is over, the missions are usually still along the lines of finding an item that allows you to move on, like a C4 or a keycard. But the last chapter does have more interesting objectives and level structure, which is nice to see. And I do appreciate the relatively short length of the missions, which fits the intended portable experience.
Playing this makes me appreciate these kind of portable spinoffs more. Back when portable systems had limitations other than just a lack of power, the difference in designing a game for handhelds compared to console/PCs were much more obvious. Liberation is a good example of a spinoff that respects the best qualities of the mainline games, but succeeded at being a good handheld game first and foremost.