Your goal in this game is to destroy as many enemy alien ships as possible and to survive their very heavy onslaughts. Enemies always appear in the background where they fly very fast at first before entering the battlefield where you can shoot them down.
Most of the enemy ships fire only one bullet that goes in the direction they are facing. But, as you progress through each level and mission alike, you will meet faster, stronger and more durable enemies which can either withstand a lot of damage (including the damage from extra weapons), or fire multiple bullets at once, or fire at your current location, or all of these at once. Such enemies are known as "Demonic Spiders" (Demonic Spiders is a nickname for extremely problematic, annoying, powerful and dangerous normal enemies that originates from TV Tropes site), and they must be quickly eliminated before they can completely disrupt your strategies in taking them out and cause you frustration.
On the "Easy" difficulty, durability of the enemies is weakened, but the amount of points they award is reduced to the point some enemies don't even award points when killed at all. On the "Medium" difficulty, durability of the enemies is normal and they award a normal amount of points when killed, although some enemies still don't award points when killed at all. On the "Hard difficulty, durability of the enemies is strengthened, to the point where even most powerful of extra weapons have difficulties to kill them, but the amount of points they award is increased to the point all enemies award points when killed.