A vast beam matrix has enshrouded the Earth. This glowing blue grid forms the Restrictor Shield, 99 sectors deep and teeming with hostile alien life.
The player’s mission is to move through all sectors, destroying the enemy craft as they progress.
Deeper sectors reveal new enemy types, each with their own method of impeding success.
You control a spaceship that flies towards the horizon on one of five beams. The aim of every level /sector is to shoot 15 white UFOs; if this is done you need to destroy the mother ship, which passes at the horizon and only can be reached by torpedos.
In later levels new enemies are added and the game gets faster. Later enemies can only be destroyed by torpedos; as there are only three torpedos per level, you will probably avoid most of those enemies.
Every enemies has its own characteristics. The white UFOs get more aggressive with the time, change the tracks more often, throw mines or ram the player.
The mother ship is protected by green blockers, that get more level by level, so it gets harder to reach the mother ship with the torpedos.
There are two extra lives per level (yellow triangles), that do not need to be shot but collected.
The game can be played with up to four players, in turns after losing a life. At the start of the game the difficulty level can be chosen, by what you can start directly in a higher level.
White Enemy Saucers
These are the main enemies the player will encounter. There are 15 in each sector, and every one will need to be blasted to progress to the next level.
Saucers change up their attack pattern every few sectors e.g. dodging from left to right while spewing a string of bombs, or simply flying towards the player’s ship in an attempt to ram them.
Once all saucers in a sector are destroyed, a Sentinel craft will appear, giving a chance to destroy it. Saucers appear from Sector 1.
Brown Space Debris
This rock-hard refuse will fly in a straight line down a beam at the player. Normal laser fire will not damage it, so it can be avoided or destroyed with one of the three torpedoes given at the start of each life.
Space debris starts appearing from Sector 2 onward.
Yellow Chirper Ships
These mouthy aliens will announce their arrival with a twin-trill alert. They will then fly horizontally from one side of the screen to the other.
Chirpers do not fire at the player’s ship, and cannot collide with it. As such, they serve merely to provide bonus points for any player with a quick trigger finger.
Chirpers start appearing from Sector 4.
Green Blocker Ships
Nigh-indestructible enemies that will streak in from the side of the screen, locking onto the beam that the player’s ship is traveling down before trying to ram them. They must then either be avoided or destroyed with a torpedo, as they are impervious to regular laser fire.
Although blockers can appear while clearing a sector of white saucers, they predominantly show up to protect a Sector Sentinel.
On later levels, more and more green blockers will assault the player, attempting to thwart any effort to blow up the Sentinel by blocking torpedoes and causing the player to switch beams to get a clear shot. Blockers start appearing as regular enemies from Sector 6.
Green Bounce Craft
This rotund enemy comes from the side of the screen and bounces horizontally across from one beam to the next, hoping to land on the player’s ship to destroy it.
If it is unsuccessful, it will bounce off the opposite side of the screen to return at another random interval.
Regular lasers cannot harm it, so shoot it with a torpedo or try not to be on the beam it bounces on. Green bouncers appear from Sector 7.
Chargers will zip down the screen at the player along a random beam (not necessarily the beam the player is currently on). They have two methods of attack, depending on what beam the player is traveling on.
If the player is on the same beam as a charger when it reaches the bottom of the screen, it will collide with their ship and destroy it; if the player not on the same beam, the charger will wait for a few seconds before leaving the screen.
During this small pause there is a chance that an unwary player will move sideways and onto the beam where the stationary charger is lurking, resulting in death. While they aren’t actually destroyed by laser fire, sending a quick shot up the beam will cause a charger to bounce backwards into the distance from where it came.
Chargers begin to appear from Sector 10.
These devilish ships will track onto the beam the player’s ship is flying along and attempt to ram them. Unlike green blockers, however, trackers will change beams to follow the player should the player try to avoid them.
Initially, they will only change beam once to try and cause a collision.
However, the further into the beam matrix the player travels, the more intent the trackers will become; soon they will start to change course two or more times in their efforts.
Additionally, they are invulnerable to common laser fire, meaning the player needs to be fast to avoid them or see them off with a precious torpedo. Trackers start appearing at Sector 12.
Red Zig Bombs
The final enemy to appear is one of the trickiest. Like the Chirper, a Zig bomb will make a sound before making its way onscreen. When it does, it will find the beam the player’s ship is on and start to come towards them.
Zig bombs cannot be killed outright with regular laser fire, but they can be handled in various ways. If the player manages to shoot a Zig bomb with their laser, it will change from red to green. It will then continue its flight down the beam it has chosen, before disappearing off the bottom of the screen.
If, for whatever reason, the Zig bomb is not shot, it will travel down the beam before performing a last-minute flip across one beam towards the player’s ship.
This means if the player fails to shoot the Zig, they need to ensure they are at least two beams away from the bomb before it leaves the screen.
Of course, the third method of taking a Zig out is to blast it with a torpedo. Zig bombs will appear from Sector 14 onward.
Once all of the white saucers in a sector have been eliminated, an alarm will sound. The Sector Sentinel will then slide into view from either side of the screen. It will continue across the beam matrix horizon until it is either destroyed or leaves on the side opposite its starting position.
During its migration, the player can attempt to bring it down with any torpedoes they have in stock.
The higher the sector number, the more green blockers will flood onto the screen to prevent the Sentinel from being destroyed. A successful hit will cause the Sentinel to explode and bonus points are given depending on the number of lives the player has accrued.
Whether or not the player destroys a Sentinel, the sector has been cleared and play moves onto the next one.
Info partly from https://www.giantbomb.com