in the original game, IIRC the invasion was more subtle. When invaders reached last screen line (i.e. touched the bottom line) it was an instant GAME OVER, not depending on how many turrets you had left.
BUT there were two lines where the game was still playable if you had turrets left:
- the one before the last; this was tricky. You were with your turrets under the alien, with the turrets cannon inside the alien body. The game did not detect collisions and you were able to kill aliens shooting them. In advanced levels it was hard because aliens thrown lot of missiles, and therefore you had to play a continuos risk, shoot and escape strategy in order to avoid to be bombed.
- last line; here the aliens were committing suicide on your turret. In advanced level a strategy when an alien crossed the line was to kill as many higher aliens as possible and not to leave two aliens on last line but only one. That way, if you had some turrets left, after a suicide you could still continue playing hoping for the best.
Yes, lot of coins spent when I was young