Intrusion is a difficult game for me to review. There's a lot of things which are good about the game: it's clearly the work of a talented programmer, and it does an excellent job of creating a difficult challenge without being unfair or frustrating. And yet, it feels like there's something missing, like it's not quite the sum of its parts. There's something which is just, for lack of a better word, sterile about the game-playing experience. And the tough part is that I can't quite put my finger on what's missing. (But I do have a few theories.)
Intrusion is a straightforward shoot-'em-up sidescroller in the proud tradition of Contra, though it's a bit more modern in some aspects -- for instance, it uses the "keyboard to move, mouse to shoot" control scheme. Also, one bullet won't kill you; you have a life bar (and the occasional health pod to refill it), and you'll definitely need it. However, there are some old-school features, like save points -- you can always choose to continue at the beginning of any level you've opened, but the levels are long enough that you'll appreciate the need not to have to go all the way back to the beginning every time. You start the game with your pistol, which has infinite ammunition, and as the game progresses you acquire 3 other weapons, each of which has a limited amount of ammunition which you can replenish by looting your defeated enemies. Naturally, there's a number of bosses sprinkled throughout the game, including the particularly lethal final boss.
Anyway, technically the execution is quite solid. The graphics, especially the background graphics, are very high quality, and the game does a good job of creating a variety of interesting environments, from a mountain base to a moving train to a flying missile (not quite as insane as in Contra 2, but still pretty ridiculous). The ragdoll physics is also a step up from what you see in a Flash game, but it's a little loose -- enemies will tend to fly around an unreasonable amount after they've been killed, and they also have a tendency to land doing the splits, which makes them look faintly ridiculous. The sounds are pretty nondescript. There's a number of nice little graphic touches (watch the missile closely when it's taking off, for instance) which also make the game feel more well-crafted. (The graphic quality does come at a price, though; the game noticeably slows down on slower computers.) The comments complain about occasional glitches, but I didn't notice anything major; all I encountered was dropped ammo ending up in inaccessible places sometimes.
As I mentioned earlier, the game is definitely challenging. There are a few very simple puzzles, but they're quite obvious and quickly solved. Mostly the challenge is just in avoiding enemy bullets (and other things that can harm you), and since there's often a lot of them and they can move pretty fast, this is not an easy task. So you'll definitely get a feeling of accomplishment when (or if) you manage to beat the game; while there is a little bit of luck involved, largely this is going to be dependent on your skill. Still, with persistence I managed to beat it, and I certainly don't consider myself particularly good at this genre, so I don't think it's out of reach for anyone, either.
So what's missing from the game? First, it doesn't do a very good job of immersing you in its environment. For starters, there's no background music; I know it sounds shallow, but I think some good music would help draw you into the game more. Also, there's absolutely no plot or backstory -- you just start out arriving at the enemy base with no explanation, just the assumption that you're supposed to kill everything. Finally, the last thing that I noticed is that the pace is a little bit off. In Contra, for instance (sorry to keep going back to Contra, but it's a useful measuring stick, despite its age) there's pretty much always something to do. In Intrusion, though, there's a fair amount of dead time when you're moving from place to place, and this gives the game a kind of empty feel. I think that's what's missing most from the game, but like I said, it's hard to put my finger on it.
Anyway, there's clearly a lot of skill that went into Intrusion, and I think if the creator teamed up with a really good game writer, they could produce a truly excellent game. As it is, while this is a fun and well-crafted game, if you're like me, you'll walk away feeling like there could have been a little bit more.