I have extremely vague memories of playing this game back when I owned a PS2 20 years ago, but I know that I never finished it, because that entire stack of games was stolen during a house party when I was in college, along with Time Splitters and Aqua Aqua. I remember being confused about why they would make a FPS game without any kind of jumping mechanic and I still think it’s a goofy idea to this day. The shooting itself isn’t all that satisfactory, the bad guys are all bullet sponges, and the story is just a bare wisp of something interesting. They take no time to build the world and don’t make an effort to tell you about anyone’s motivations other than “the bad guys are invading, you have to stop them!”. I think there could have been a good game in here somewhere, but it never makes it to the surface of what actually gets shown.
An aside, I got this game in the “Killzone Trilogy” which was ‘remastered’ for the PS3 and in order to play this, the first game, you put in the Disk for the second game, go to the extras for the second game, install the first game, then play it from there. It’s clunky at best and I hated the experience. Hopefully the second game is more better?