The problem with attempting to answer this is that Game of Thrones, by its very nature, is a fantasy novel, not a representation of any one time in human history. I agree that George R.R. Martin largely drew inspiration from medieval history, but he also doesn't seem to have felt any compunctions against mashing up places, events, and even time periods, radically altering chronologies, and inventing new institutions/systems (the Masters or whatever they're called, ravens, 1,000-foot walls, etc). As well he should! He's writing speculative fiction, not scholarship, and doesn't make any pretense of doing otherwise. The 15th-century War of the Roses seems to have been a particular inspiration, but it is idle to attempt a close comparison between an invented world and our own.