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Invasion Force was made in 1984 by Micromania for the ZX Spectrum. The game was made to be played with the Stack Light Rifle, which was the very first computer light gun. Invasion Force is one of the very few ZX Spectrum exclusive light gun games. The game requires at least 48 KB of RAM. If I load the game from audio to my + 128K version of the ZX Spectrum the game runs but has some display errors while showing the score screens.
In the game's story the player is using an anti-plasma laser to protect cities and laser bases from an alien invasion force. The aliens attack in waves. During a given wave all the enemies are of the same kind. In total there are 35 unique enemy types. The aim is to keep many cities for as long as possible, as each city grants 100 points upon conclusion of a wave, whereas a hit enemy just yields in 10 points. Every 10'000 points a broken city is re-built. Each round the player has a disposal of 40 shots which is 20 shots per laser base. The laser bases are the green circles at the borders of the screen. If an enemy crashes into a laser base, the ammunition of this base is lost for the current round. The difficulty of the game increases each round.
Visually and conceptually Invasion Force looks and plays very similarly to the 1989 game Missile Ground Zero. This is especially true for the ZX Spectrum version of the game made by Software Creations. Personally I think Invasion Force is the better game, as for the superior hit detection and the more appealing graphics.
Among the Stack Light Rifle compatible games for the ZX Spectrum Invasion Force is by far the best playing and the best looking game. The hit detection feels orders of magnitude more reliable. The graphics are very pretty and the animation of the enemies is fluent. Personally I like the sound effects a lot and I think they are fitting the game nicely.
The cover represents the game very well. It shows the two laser bases, invading aliens and the anti-plasma laser as it protects a city.