Originally the idea was to have every other epiosode a purely historical one - in fact the first story (An Unearthly Child) was one of these - set in prehistoric times. After that came the first Dalek story, then one about Marco Polo. The Aztecs and the French Revolution story were the other historical ones in the first season.
By the second season the appeal of the alien stories was becoming clear & that season only had two purely historical stories, the Romans and the Crusaders, although there was one set in 1066, but it turned out that events were being manipulated by a rogue Time Lord - no, not the Master, but the Meddling Monk.
Season 3 again had two historical stories, the Trojans and War of God, about the wars of Religion in 16th century France.
In Season 4 we had the appalling OK Corral story & one about smugglers set in 17th century Cornwall, and then after the Doctor's regeneration into Pat Troughton, the Highlanders, which brought Jamie (Frazer Hines) into the Tardis and was the last purely historical story until Black Orchid, a Peter Davison story set in the 20's