Zombie Zombie is a ZX Spectrum computer game by Sandy White. It was published in the 1984 by Quicksilva. It is the sequel to the popular Ant Attack, by the same author, and uses the same 'Softsolid 3D' isometric graphics.
Like Ant Attack, Zombie Zombie has a B-movie storyline. From the game instructions:
Words were spoken at the setting of time. Words which put together told a terrifying tale, a prophecy of doom. Man had crossed into the black land and returned diseased. Evil had found a door into another Kingdom. Somehow it had to be kept shut. Prehistoric man kept vigil until it was forgotten why. Of course they thought you were crazy, but you insisted on returning to that ancient place, just to make sure. You had a feeling about these relics, a feeling of death. Unwittingly you had sprung an invisible lock of an invisible door. A prophecy is about to be fulfilled. The Dead will rise again to eat the flesh of the living.
Zombie Zombie had the inclusion of a helicopter which could be piloted and used to alter the terrain. To kill the zombies you had to make them follow you to a high point then let them fall off the edge.
Zombie Zombie also presented two notable innovations. The first was the utilisation of two-channel sound, a then unheard-of feat of a computer equipped with only a single-channel piezo buzzer as its sound source. The second was that, through the use of the Sinclair Interface 1, the game's two-channel melodies could be played back on a MIDI-equipped synthesizer. Unfortunately, although a wiring diagram was supplied with the game, the interfacing code had been tested on a Yamaha DX7 with a ROM containing bugs - so the circuit would only work on that particular synthesizer. This came to light when Sandy White attempted to demo the game while connected to an unfamiliar synthesizer.
Source: Wikipedia, "Zombie Zombie", available under the CC-BY-SA License.