they only go one way. down.
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Zap the whole block before it lands on you. The fewer are left, the faster they come — so the end of every wave is the hard part.
Press ◀ or ▶ to slide, press FIRE to shoot. You can also drag your thumb anywhere on the screen, which fires as well. On a computer it is the arrow keys and the space bar.
Only one of your shots can be in the air at a time, so a wasted shot really is wasted. Wait for it to land.
Hide behind a shield — but your own shots chew holes in it just like their bombs do, and an invader that gets low enough eats it completely.
A zapped invader sometimes drops a power cell. It falls straight through the shields, so getting one means coming out into the open. Worth it.
The saucer is worth 50, 100 or 150 and you never know which. Hit it while a shield is damaged and it drops a patch that mends one.
With three left they stop marching and run away from you, and they still throw bombs while they do it.