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'.