Space Assailants is an obvious Space Invaders clone, with some slightly different twists. Instead of avoiding shots fired by the invaders, the player must dodge a starfield that constantly scrolls vertically from top-to-bottom and repeats. The game starts with only one row of invaders. After you clear the first set, the next set requires the player to shoot down two rows of invaders. On the next set, a third row of invaders arrives. The pattern continues until there are six rows of 6 invaders once you reach the 6th level, but the characters in rows 4-6 are a repeat of the same characters used in rows 1-3.
The invaders move across the playfield until they hit one end of the screen, then they drop down a level, reverse direction, and head the other way. The invaders are arranged in a hovering position above their spaceship. Once the spaceship lands at the bottom of the screen, or the player loses all three lives, the game is over.
The status banner at the top of the screen indicates the number of lives, the score, and the level. Once the game is over, an end-of-game screen shows you this information and lets you start anew.
Space Assailants was primarily written by John Weisgerber and really pushed the envelope with what could be done with Atari Basic. The scrolling stars are actually in a different display list than the invaders, and the two screens are alternated very rapidly (every 1/60th of a second) so the eyes perceive them to be on the screen at the same time. In the screenshots above, only either the invaders or the stars are visible, so imagine the images combined to envision what the player is experiencing. There is a flickering effect (like the one criticized so heavily in the Atari 2600 Pac-Man), but the flickering effect wasn’t as dramatic on old raster-scan televisions.
The banners on the left and right edge of the screen are Player/Missile graphics. The ship and its shot are also Player/Missile graphics. The stars are low resolution graphics pixels. The invaders are re- defined characters in multi-color mode, and they animate with every move in two different poses. The game makes use of the typical droning vibrato background sound effect used in may A/W/A games to take the place of a background melody.
Eric originally wrote an assembly routine to be run during the vertical blank interrupt that would smoothly move the ship left and write based on joystick controls. It worked perfectly except for one problem: John’s technique of allowing for interleaving the two graphics modes disabled the vertical blank interrupt, so you couldn’t do both! The game as it stands uses BASIC to move the space ship and so it is not as smooth as it would have been in assembly language.
The technique used in this game to interleave two graphics display modes was definitely not used in any other BASIC game or any other game that the A/W/A Team is aware of!