The System reacts to the player's actions. In its simplest sense, a player that is highly aggressive and wastes ammunition will face tougher opposition since the System will react as if it is facing a larger fleet. Since player actions largely dictate the type and number of ships you will face, a level may change dramatically on each pass through depending on how you play the game. This increases the challenge and replay value, especially considering all the secrets to be discovered and variety of weapons available.
The number and strength and types of opponents you face varies depending on the aggression levels of the System. When the aggression level goes up, so does the strength of your enemy and the degree of challenge you face in surviving.
The following actions appear to increase the aggression level:
- Contrary to popular belief, destroying a lot of enemies will increase the enemies more than shooting a lot, especially destroying them with your standard gun. Aggression levels seem to increase faster and the more powerful the weapon being fired.
- Upgrading certain Special Weapons, especially picking up Special Weapon 2 or upgrading Special Weapon 3 a lot. New enemies which spawn that way are able to appear in almost any place, even when fighting Capital Ships or Fortresses.
- Failing to destroy a base in the allotted time.
- Staying alive for a long time.
The following actions appear to decrease the aggression level:
- Destroying a base before the timer runs out.
- Losing a Zanac.
- Using an Enemy Eraser, Weapon 6 or Fairy. Most notably it stops various "floods" of enemies (lots of enemies at predefined moments) or waves of enemies spawned after picking up certain special weapons.
- Reducing your rate of fire for a while, or increasing your reliance on your Standard Gun.
- Destroying a Sart.
There are 2 types of ways the enemies spawn: 'Dynamic' and 'Scripted'.
- Some events on each level are 'scripted' or 'predefined' -- for example, near the beginning of Level 3 you will always be attacked by a group of type 2 Degids regardless of the aggression level. However, the entrance positions of "Scripted" enemy spawns can vary and are often determined by the position of your ship. For an example, if in the beginning of level 1, you move to the right side of the screen, a Sart will appear on the left side of the screen and vice versa.
- On the other hand, when you see Type 3 E-Fains or the larger non-capital ships like Mortars, Yellow Logas and Golgos in places where they don't otherwise appear, you can assume that the Aggression Level is very high. Some of the 'Dynamic' enemy patterns won't reset completely after dying, destroying Sart or even if the game is over and new game is started. To reset the AI completely, a game reset must be made.
The number of enemy projectiles on the screen is limited. When the number of bullets exceeds a certain number, some of the enemies start to fire less frequently or not at all. For an example, when 2 Combo Cores start to fire constantly, then one of them fires much less frequently or stops firing altogether until the other combo core is destroyed. Sometimes the projectile limit causes some of the projectiles to disappear automatically.
Periodically the scroll of a level will halt and you will be given a time limit in which to destroy a large fortress, an assembly of cores. If you destroy all of the cores before the time runs out, the screen will be cleared of enemies and you will receive a bonus before the scroll continues or you move on to the next level. If you don't destroy the base in time, the aggression level of the System may increase. In addition, while starting to move away from the fortress, all its remaining turrets fire at full force which is especially dangerous since screen starts scrolling again and therefore many bullets will be fired upwards. Destroying all cores at that moment won't give you base destruction bonus. If it's the final fortress, you'll gain invulnerability as you fly away though.
A typical base has at least four or more cores. Initially, the cores take turns firing in a pattern. As you destroy the cores, the remaining cores will begin firing more often. Finally, when only two to three cores remain they will all fire continuously. This can be a problem if the remaining turrets are difficult to deal with based on your level of weaponry.
For example, some weapons are unable to destroy Leads, so Slit Cores and Combo Cores should be destroyed first unless you are using a weapon like Tile 7. Round Cores, which fire round blue hard to destroy bolts known as Beruos, are also difficult to deal with when they fire continuously. In general, you should save turrets that fire only Light Bars for last since any weapon can easily remove those projectiles.
If you end up with a Lead or Beruo core as the final surviving core, it can fill up the screen with so many Beruo that you are unable to reach it with your weapons. Try maneuvering into the upper left or right corners of the screen. You may be able to hit the core a few times sideways with a weapon like Tile 0, but more importantly the Beruos will follow you. After they are all heading for the corner of the screen, dodge back underneath the core and enjoy a few precious seconds of clear shooting at the turret. Repeat this a few times and you should be able to destroy any core, as long as you don't run out of time.
Some bases have very Turret 1s and Turret 4s that shoot Leads and Light Bars. These cores are smaller than the normal fortress cores and fire constantly. These targets are optional — they can be ignored and you will still destroy the base if you obliterate all the regular cores in time. It is still a good idea to take them out at the start just to clear the field, especially since destroying them will not affect the rate of fire of the main base cores.
Another useful tactic which sometimes can be used is to approach very close to the core and fire away when it opens and retreat when it's about to fire when you're unsure if you're able to destroy the core in time. The efficiency of this tactic is greatly increased if your main weapon is upgraded by red landers. Fully upgraded main gun can destroy a core up close within less than half of a second.