In this game, which has a generic storyline about how you need to fight off an empire ruled by a mad scientist who discovered a new kind of energy called Xenos (surprise!), you control a "battlesuit" (though I look at it and see a super robot) which flies freely through a mazelike environment whilst wielding an energy-based melee weapon of some kind, intent on smashing the "Xenos Generators." (You're the little thing with the bluish aura and the sword.)
Combat is insanely simple: See an enemy? Fly at it, try to avoid any projectiles it fires, and smash into it head on. Whenever you smack into something that can be destroyed by your weapon, your robot automatically swings at it. (Note that this means you need to keep mashing the arrow keys if the thing is a generator and you're just sitting there.) This is actually quite well-animated.
Now, this sounds very simplistic, and if that was all there was, it would be. If you look at the above, you might have trouble picking out the robot. That's because it's got a different weapon (a double-bladed sword thing in the style of Darth Maul's lightsaber) and it's now green instead of blue. (It's right in the middle of that mess of enemies-you can tell because all those pink energy blasts are between it and each enemy, more or less.)
It's a different weapon because there's more than one (you start with two and acquire several more), and the color's changed because of the other central part of the combat system: The more damage you do, the more that large bar that's shaped like three donuts in a row fills up, and the larger and more powerful your
Now, here's the tricky part. That energy buildup drains over time (and rather quickly when you're not in battle), and so you need to constantly be fighting, or you'll run out of energy; further, each weapon has a "special" attack that you activate by pressing the space bar which also drains energy.
That double-bladed sword is by far the better and more fun of the two starting weapons, and even gives the better of the two other weapons I've seen (a cool scythe thing) a run for its money. Why? Aside from the fact that it just seems to work better and more smoothly most of the time, especially against large numbers of enemies (which are the norm), its special attack is a boomerang throw.
This is awesome, because you can throw it into a cluster of enemies, who will take lots of damage as it goes through them, and then fly through another cluster of enemies; the weapon will follow you wherever you go, slashing through whatever enemies you happen to pass by as you run around, and keeping your energy charge much higher than the other weapon specials do. (Note that you're more vulnerable when you do this because you don't smash guys you fly into, but you're also more maneuverable, because you slow down when you're slashing through enemies.) There is very little more satisfying than executing this maneuver well.
The addition of aggressive mines (which you can't destroy except by being hit by them or by outmaneuvering them so that they smack into walls, another thing that is very satisfying when you do it well), random energy/health powerups, and barriers that trap you in with enemies who sometimes come in multiple waves add many strategic layers to the game; over time, you'll realize that it's good to save those powerups for when barriers fall (unless you really need some health), that it's better to destroy the generators you can reach first and then fight the enemies, and that you need to choose where you fly carefully to avoid waking up those darned mines.
All these things plus the great graphics add up to a game that's a lot of fun and very engaging.
That said, they also add up to a game that's very hard. Enemies fire their weapons in salvos, which means that you can lose health at frightening rates, going from a full health bar to dead in seconds, especially if you've crashed into a huge mass of them as is so very tempting, and the bosses can spray enough projectiles to make anyone but a bullet hell player flinch (though thankfully, they usually have nice, predictable patterns). It occasionally gets to the point where the levels where you have to deal with the advancing laser beam of doom that crosses the whole screen are relaxing.
It's rare for me to really enjoy a game where that kind of stuff is true, but you know what? I love this game, even if there's a good chance I'll never be able to drive myself to finish it (much less the distinct possibility that I'm actually incapable of finishing it). If you're a gamer who loves fast-paced actiony titles like shooting games, this is probably right up your alley. If you don't love that kind of game, you'll probably still have some fun, but might find yourself running out of patience quickly.