Isometric 3D was a popular graphical technique for computer games during the 80s. Introduced to the world with Sandy White's 1983 game 'Ant Attack', and perfected by Ultimate's 1984 'Knight Lore', the technique (also known as '3/4 perspective') involved rendering sprites from 45 degree angles above and to the left or right, such that a cube (for example) appeared hexagonal. Think 'Marble Madness' and you're there.

The technique was particularly popular with platform games ('Head over Heels' being probably the ne plus ultra of this generic sub-set), although Bullfrog and The Bitmap Brothers repurposed it with 'Populous' and 'Cadaver' respectively.

Like so many other things, it was initially very popular as a novelty, after which it became unfashionable, after which it assumed its position as a valid part of the game designer's pallette. Isometric 3D died a death in the late-80s and early-90s, almost killed by the resurgence of the side-on platform games it had replaced. During the mid / late-90s it reappeared in a few key games, most notably 'SimCity 2000', 'Civilization II' and 'Diablo'.