Isometric Projection

Tiles and sprites are drawn as parallelograms at a constant angle, usually with no perspective foreshortening, and objects are sorted back to front so nearer things overlap farther ones. Since the camera never rotates, every asset can be drawn once by hand, and the world remains a grid the code can reason about. Trade-offs show up as ambiguity about exact height, occlusion behind tall objects, and diagonal movement that fights the controls.

Sega's Zaxxon in 1982 flew a ship over a fortress rendered at an angle, using shadows to communicate altitude in a space where nothing else could. Same-year arcade games such as Q*bert reached for the projection too, so the exact first is debatable; what Zaxxon showed was that the angle could carry gameplay information rather than just decoration. Drawing at a slant cost nothing extra on 2D sprite hardware.

Projection became the default for games that need to show a lot of state at once: cities, battle lines, dungeon floors and party positions all read better from above and to the side than from eye level. Real 3D never displaced it, because a fixed angle guarantees readability and consistent art. What it gives up is intimacy and precise spatial judgment, which is why action games mostly left it behind.