Defender is the first of the true classic space horizontally scrolling shoot-em-up games, with its complex control system, radar scanner and frenetic action.

Your mission is to protect the humans, which are carried away by the landers. You start the game with 10 humans to defend and bonuses are awarded at the end of each attack wave for the surviving humans. If all humans are killed the planet is destroyed and you enter null space where all landers become mutants, and your chances of survival are greatly reduced. You will then have to survive until the humans are replenished.

You are armed with two types of weapons. Firstly, your ship is equipped with a fast firing laser which destroys any enemy ships which it hits. You also have smart bombs which when detonated destroy all enemies on screen (usually). Lastly you have a defensive weapon, the hyperspace jump, which instantly takes your ship away from danger and rematerialises it elsewhere above the planet. Hyperspace however has a high risk factor, and can result in the destruction of your ship. Only use it as a last resort.

You are provided with a radar scanner which gives a mini view of the whole gameplay area. Your position is marked by a large white dot, the humans on the surface by smaller white dots and each enemy ship by colour-coded dots.

Your adversaries come in the form of 6 different types of enemy ship, each with their own attack patterns and characteristics. The lander is the basic enemy ship which moves around the screen firing occasional bullets at you. Its primary mission is to abduct the humans which it does by picking them up and carrying them away. You must the destroy the lander without hitting the human, which causes the human to fall back to the ground. Humans can only fall a short distance and survive, so you should move in and catch them and deposit them safely on the surface of the planet, which will award you 1,000 bonus points.

If a lander reaches the top of the screen carrying a human, the human and lander are combined to form a mutant. Mutants are a lot more deadly than landers, homing in on your ship firing a lots of bullets.

Level 2 onwards sees the first appearance of the other three enemy ships. The first of these is the bomber, a slow moving ship which moves steadily across and up/down the screen leaving a trail of mines behind it. These usually appear in a group of three, and do not pose a great threat. They are more of an inconvenience, getting in your way when you are otherwise engaged in a fast moving battle with a couple of mutants and four swarmers!

The pod is the most valuable ship in the game and one appears on wave two, two on wave three, and four on every wave after that. A pod when destroyed by laser fire breaks open to release swarmers.

The swarmer is very small and tricky to hit with laser fire. They move quickly around firing occasional shots are are best picked off as soon as they appear.

Take too long to complete a level, and you will be joined on screen by a baiter, the fastest moving and most deadly of all your opponents. They home in rapidly on your ship, firing a stream of bullets, and prove tricky to avoid, as they cannot be outrun.

You begin the game with three lives and three smartbombs, and an extra life and smartbomb are awarded for every 10,000 points.

At the start of every fifth attack wave the humans are replenished to ten, and null space is ended.

Try to use your smartbombs wisely. The more enemies on screen, the more points you score when detonating a smartbomb. If you use a smartbomb to destroy a pod, you will also be awarded points for killing the swarmers which would have appeared.

When using a smartbomb to destroy pods, try to remain stationary, otherwise swarmers may escape. The big scores come from destroying all pods on an attack wave at the same time, when you can often reach the next extra life and bomb award.

Swarmer reverse line - there is a position above the planet which swarmers cannot move past. Cross this line and the swarmers turn around and move towards you in the opposite direction. Use this line to lure swarmers to their death.

Baiter reverse line - there is also an invisible line which baiters cannot cross. Use this to escape a pursuing baiter.

Reverse several times in quick succession to confuse a pursuing baiter, giving you about one second to destroy it while it will not be firing at you.

You can save yourself from the deadly null space by catching a human and not depositing him back on the planet's surface. Simply save him until the end of the attack wave, and do the same again on successive levels until the humans are replenished.

Between 990,000 and 999,975 points an extra life and smartbomb are awarded for every enemy destroyed.