The race to put new home computers on the market is matched only by the software companies race to be the first to market versions of the "standards" - Invaders, Asteroids, Defender, etc for each new machine. There must be a strong temptation to put speed before quality, but fortunately Quicksilva have produced two arcade games for the Spectrum as good as anything they produced for the ZX81.
Space Intruders is the best version of Invaders that I have yet seen on any home computer. There are three types of invaders, plus a wobbly "mutant" which you can try to zap before it turns into an ordinary invader. Each hit is marked by a little starburst, including ones on the command ships which cross the screen in both directions.
The game gets progressively harder by speeding up and by allowing the invaders to drop a greater density of bombs at you. The bombs fall at two speeds, and you have to be on your toes to avoid the quick ones. There are several features to help you win a high score - the first button "auto-repeats", an extra base is earned at 50,000 points, and if your base does get hit, all the invaders' bombs are cleared from the screen to give your new base a fighting chance. In addition the game can be "frozen" on the screen by pressing H and then continued by pressing S. We didn't have this kind of luxury in my day - you needed endurance to win a high sore then! Unfortunately the program does have one bug - every hit scores 660 points whatever kind of invader is knocked out. I found this rather spoilt the game for me as it made me change my tactics since there was no point in chasing mutant invaders.
Excellent use is made of the limited Spectrum sound effects - by using warbling rather than steady tones there is no slowing of the action. On-screen scoring and highest score, plus a high-score table, full use of colour, and "self-play" demonstration mode made this a really excellent, full-featured Invaders.
Space Intruders is £5.95 from Quicksilva, 92 Northam Road, Southampton SO2 0PB.