Philips Magnavox Videogames and the Entertainment Revolution Trigger Happy - Manual (Page 213)

Trigger Happy
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Fig. 10. Zaxxon: isometric perspective and terraformed space
(‰ Sega 1982)
however, while having as usual to deal with enemy
aircraft, could also explode if it crashed into any of the
numerous barrels, pylons and buildings poking up out
of the ground. Movement was now nearly in three
dimensions, with the introduction of controls to vary
“height” above the ground. Only the fact that motion
was automatically one-way (a function of the scrolling)
inhibited complete freedom of movement.