At the height of Zarta's fully automated society in the 22nd century, their automated factory produced one of the first city builders. It was seen by the people as a great triumph, one of them most complex machinery ever built. In less than a month, it was to construct a city to the people's expectations. Unfortunately, the city builder went berserk, building more cities. The other machines then began to go berserk, supplying the city builder with more material.
In 2266, the city builders were close to completely covering the planet in cities when Captain James T. Kirk of the USS Enterprise arrived. After the first two attempts to stop the city builders failed, Spock analyzed the metal of which they were made and determined that a type of acid would dissolve it. With the aid of the Enterprise crew, the people of Zarta finally destroyed the city builders. (TOS comic: "Invasion of the City Builders")