Zektor was released in August 1982.
The last alien is the alien from 1981's "Space Fury
There are hidden letters in each of the eight cities. It is not known what they mean, but it is speculated that they are either the designers initials, or part of a secret code from the programming.
The The eighth (and last) alien you fight is the alien from Space Fury. The eighth round will repeat indefinitely.
As with all of Sega's vector games, the Electrohome color vector monitor used for this game has a notorious tendency to catch fire. It is unknown what to do to prevent this.