London is in peril! An Interplanetary Battle Fleet from Mars whose inhabitants are infuriated by our claims that their planet is dead – has attacked. The Ministry of Defence is asleep, so you have to defend the city with just one missile launcher.
You can move the launcher to the left using capital Z and to the right using X. Press F when you are ready to fire.
The aliens descend cautiously at first, but be on your guard. If you let a single ship through, London will be devastated.
During the game you score points for each invader you hit more points for a ship high in the sky than for one close to the ground.
If you want to make the game more difficult, try replacing (N +2) in line 200 by (N +1) or reducing the 4 in line 290 to 3, 2 or 1.
PROGRAM NOTES
Lines 100-150: Define ‘invader’ and ‘building’ graphics, then set up display colours and initialise variables.
Lines 160-190: Display various buildings of random height.
Lines 200-280: Move a randomly selected alien ship, checking to see whether or not it has landed on a building.
Lines 290-320: Check the keyboard and move the launcher if Z or X has been pressed.
Lines 330-380: Fire the missile. The computer jumps to line 390 if a ship is hit.
Lines 390-410: Calculate the new score and determine which ship has been hit by the missile.
Lines 420-450: Make a short sound and rearrange the lists of alien positions stored in arrays Rand C. If there are no aliens left, the computer jumps to line 490.
Lines 460-480: Display an alien ship landing and a suitable message.
Lines 490-500: Print the final score and stop the program.