Alien Defense

A block of aliens marches sideways across the top of the screen, drops a row at each wall, and comes back. You have a laser cannon at the bottom, three lives and four shields that erode as they take fire from either side. Clear the formation and another arrives, usually starting lower than the last; let it reach the bottom and the game is over.

An Accident That Became the Design

In the 1978 arcade original the aliens accelerated as you shot them, and that was not a design decision at first. The hardware could only redraw so many sprites per frame, so a full formation took a long time to update and a nearly cleared one took very little. Fewer aliens on screen meant a faster pass. Tomohiro Nishikado noticed the effect, judged that it made the game better, and kept it. It is an unusually clean case of a limit in the layer underneath leaking upward and becoming a rule of the world above it, which is normally the thing a good physical description is built to prevent.

How to play

• Left and right move the cannon; Space fires one shot at a time, and P pauses.
• Your own shots eat holes in your shields, so pick where you fire through.
• The saucer crossing the top is worth more than anything in the formation.
• The last few aliens move fast enough that you should aim where they will be.

Read more: Effective Field Theory