"Space Invaders" pitted the player against multiple enemies descending from the top of the screen at a constantly increasing speed. The game used alien creatures inspired by "The War of the Worlds" because the developers were unable to render the movement of aircraft; in turn, the aliens replaced human enemies because of moral concerns (regarding the portrayal of killing humans) on the part of Taito. As with subsequent shoot 'em ups of the time, the game was set in space as the available technology only permitted a black background. The game also introduced the idea of giving the player a number of "lives". It sold over 360,000 arcade cabinets worldwide, and by 1981 had grossed more than $1 billion, equivalent to $2.5 billion in 2011.