Tara was a planet colonised by humans who chose to pattern their culture on late medieval customs, with a rigid monarchical hierarchy.
The Taran nobility took pride in their scientific and mechanical ignorance, leaving such matters to the 'peasants'. Their primary ranged weapon was a crossbow that fired electric bolts; this was considered a peasant's weapon.
Centuries before the arrival of the Fourth Doctor and Romana I, a plague wiped out 90% of Tara's population. During this time of strife, the peasants were largely abandoned by the ruling class and manufactured large numbers of androids to cope with the labour shortage. (TV: The Androids of Tara)