It would be partially inaccurate, and dishonest, to call Game of Thrones a period drama. It doesn't take place in any known period of Earth's history, or on Earth for that matter- the show does not explicitly refute this but it is still highly unlikely. It does, however, appear to take place in a fantasy setting influenced by our own world and its history. The prime influence is the Middle Ages or the Medieval period. We can see this in one of the regions called Westeros, an island modeled on the British Isles. Geographically and culturally it is identical to medieval England; just think of it as Britain with different names for locations. Interestingly, the accents in Westeros remain faithful and accurate to their locations in actual Britain. Characters from the North of Westeros have northern English accents. The central characters, The Stark family, have Yorkshire accents, mostly. Sometimes the actors get to use their real life accents; Sean Bean, who plays Ned Stark, is himself from Sheffield, England, home to the Yorkshire accent.
Go further north, and the characters speak with Scottish accents and so on. The South of Westeros features the English accent that is familiar to everyone, the Queen's English (also known as Received Pronunciation), an accent tied with the upper class and regality.
The social hierarchy portrayed in Game of Thrones is perhaps its closest tie to the medieval period. It is essentially the feudal system of medieval England transplanted in a fantasy world, and it remains virtually unchanged. The feudal system is probably what defines the medieval period for most people, maybe after images of soldiers clad in armor, swordfights, and kings and queens, etc. Although, these examples are lent support by that system.
The basics of this system are fairly simple. The people...