The player flies a small spaceship above a scrolling, mountainous landscape which wraps around, so flying constantly in one direction will eventually return to the starting point. The player's ship flies in front of the landscape and does not contact it.
The terrain is inhabited by a small number of humanoids. Enemy ships fly overhead. The goal is to destroy the enemies to prevent the humans from being captured.
The player is armed with a beam-like weapon which can be fired rapidly in a long horizontal line ahead of the spaceship, and also has a limited supply of smart bombs, which can destroy every enemy on the screen. The player also has a limited supply of "Inviso" cloaking energy, which makes the ship invisible, and able to destroy any ships it comes in contact with.
At the top of the screen is a mini-map, which displays the positions of all aliens and humans on the landscape.
Aliens[edit]
The aliens in Stargate. Most of the names of the new types are inside jokes.
There are fifteen types of aliens:
Lander - The primary enemy on every level. Landers teleport into the level in staggered waves, and attempt to capture humanoids by descending upon them and dragging them into the air; if they make it to the top of the screen with a human, the two fuse together into a more dangerous Mutant. Landers can fire projectiles at the player.
Mutant - A mutated Lander. Mutants home in on the player at constant speed, firing projectiles. They move erratically, making them difficult to shoot.
Baiter - A flat, iridescent spacecraft that teleports in if the player is taking too long to complete a level. Homes in on the player and attempts to match their speed, whilst firing accurate projectiles. A difficult opponent due to its unbeatable speed and tiny horizontal cross-section, which makes it very hard to shoot.
Bomber - A box-shaped alien that lays stationary mines in the air.
Pod - A star-like alien that bursts into a number of Swarmers when shot.
Swarmer - A tiny teardrop-shaped alien that moves very quickly in an undulating fashion. Difficult to shoot.
Firebomber - A rotating variation on the Bomber, which shoots high speed Fireballs at the player.
Yllabian Space Guppie - An undulating attacker, which attacks in swarms and homes in on the ship.
Phreds and Big Reds - Square aliens which look like they are constantly opening and closing their mouths. Similar to the Firebombers, they launch tiny versions of themselves called Munchies.
Dynamos - Diamond shaped ships composed of clusters of Space Hums, which periodically break off to attack the ship independently.
Once all aliens (except Fireballs, Space Hums, Baiters, Phreds, Big Reds and Munchies) are destroyed, the player progresses to the next level.