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The formation is four rows of ten, and that is the entire game. Unlike almost every other descendant of the arcade original, this grid never marches downward. It slides about a hundred pixels to the right, slides back, and repeats for as long as you are alive. Nothing is closing in on you and nothing is speeding up.

That one decision changes what the game is about. In the classic version, the invaders' descent is the clock - you are racing them. Here there is no clock at all. You can take as long as you like, which means the only thing that will ever kill you is the volume of fire coming down, and every death is one you had time to avoid.

Clearing all forty ends the game with a win rather than a high-score screen, which is genuinely uncommon in this genre. At ten points each, a perfect run is four hundred.

Every two-tenths of a second the game picks one living alien at random and fires from it. Not the nearest one, not the lowest one - a random one from whatever is still on screen. There is no pattern to memorise and no safe column, which is why the difficulty holds up even though the formation itself is static.

The shot is then aimed directly at wherever your ship is standing at the instant it leaves. That is the single most useful thing to know about this game. The bullet is not tracking you as it travels; it committed to a heading the moment it spawned. Move after you see a shot appear and it will pass through where you used to be.

Their bullets also travel at less than half the speed of yours, so you have more reaction time than the screen suggests. What makes it dangerous is quantity: they fire faster than you do, and up to thirty of their shots can be in the air at once.

Your ship reloads on a fixed interval of three-tenths of a second, slightly slower than the aliens fire, and only ten of your shots can exist on screen at any moment. Fire into empty space at the top of the screen and those rounds stay in flight for a while, and during that window nothing new comes out of your ship.

In practice this only bites when you panic-fire at a distant row while something is descending on you. Aiming at the lower rows keeps your shots short-lived and your gun effectively always available.

There are two touch schemes running at once. Arrow buttons sit at the bottom of the screen, and you can also swipe anywhere on the playfield to steer, with a tap acting as the Spacebar. Both work, and the swipe option is the more comfortable of the two once you stop looking at the buttons.

It is built on Phaser and scales to whatever screen it is given, so the formation stays proportionally the same on a phone as on a monitor. Nothing is cropped and no part of the grid is harder to reach on a small display.