George R.R. Martin has explained his historical influences in a rather long correspondence with a fan.
From Citadel SSM Entry "Historical Influences":
Q: Firstly I'd like to apologize for wasting your time, reading this email; and I often think that you must be St. Job reborn... Where
do you find the patience to read and answer to all your fans?
George: Patience is not the problem. There's just too few hours in the day.
Try as I might, I do find myself falling further and further behind. I
still have letters in my box from 1998. Sigh. But I plug away when I
can...
Q: [Edited for clarity after this point. Ser Loras's (Ser Loras is the nickname of the fan) question concerning whether GRRM borrows from history, particularly Spanish
history, received the reply below.]
George: Well, yes and no. I have drawn on a great many influences for these
books. I do use incidents from history, yes, although I try not to do
a straight one-for-one transposition of fact into fiction. I prefer to
mix and match, and to add in some imaginative elements as well.
Most of my borrowings, however, come from English and French medieval
history, simply because I am more familiar with those than with the heroes, legends, and traditions of other countries. The Wars of the
Roses, the Crusades, and the Hundred Years War have been my biggest
influences... oh, and some Scottish history as well, such as the
infamous Black Dinner that inspired my own Red Wedding. This isn't a
matter of choice so much as it is one of necessity. I don't have any
other language besides English, and there's a paucity of good popular
English language histories about medieval Spain, medieval Germany, and
the like. I was in Germany last fall, and looked everywhere for good
reference books about the medieval Holy Roman Empire, which would be
treasure trove, I suspect. There are a ton of them that looked
likely... but all in German.
And in about a week I will be travelling to Spain, coincidentally
enough, where I plan to search for some good popular histories as
well. Whether I will find any I can read, however... well, it's
doubtful.
As to your specific question, I have seen the film version of EL CID,
of course, and the Osprey book about the Reconquest is on my sheld.
Good, but not nearly detailed enough.
Q: Also, the fight between the Baratheon brothers for the throne is similar to the one held by the Trastamaras: Pedro el Cruel (Peter
the Cruel), King of Castile and León and his brother Enrique (Henry).
Again, am I close?
George: I know a little more about that one since it impinged on the Hundred
Years War, and there are plenty of references for that in English. As
a matter of fact, I collect miniature lead and pewter knights in 54mm
scale, and I have figures of both Pedro the Cruel and Enrique the
Bastard in my collection.
Q: More... I see a lot of Henry IV of Castile in Robert Baratheon, if I am right, you will now what I mean...
George: Sorry, Henry IV is not a fellow I know much about. If Robert is
modelled on anyone, it is more Edward IV of England... though as
usual, I rang in some changes.
Q: Also, could Don Beltrán de la Cueva be similar to Ser Loras Tyrell. I mean, their histories do not fit perfectly but Don Beltrán
(First Duke of Alburquerque) was reputed to be the best knight of
Castile by then, and his sexuality raised many questions.
George: Again, don't know him. Wish I did. If I could find a good book...
Q: and lastly, there could be a parallelism between Aegon the Conqueror, and the spanish "conquistadores". Let me explain: Hernán
Cortés for example, with less than a hundred men conquered the Aztec
Empire... (Dragons=gunpowder?)
George: I know about Cortez, but Aegon the Conquerer derives more from William
the Conquerer.
I would love to become more familiar with Spanish history. Can you
recommend any good English language popular histories? I stress
"popular." I am not looking for academic tomes about changing patterns
of land use, but anecdotal history rich in details of battles,
betrayals, love affairs, murders, and similar juicy stuff.
Q: Thank you very much for your time, and I await with illusion for a reply.
George: I hope you didn't wait too long. Here 'tis.
Keep reading.
As you can see, here George has listed almost all that has influenced him:
- War of Roses
- Crusades
- 100 Years war
- Black Dinner
- Edward VI
- William the Conqueror
- General English and French medieval history
George has also listed other influences which are following.
Hadrian's Wall has inspired The Wall. From a 2000 Interview:
Well, some of it will be revealed later so I won't talk about that
aspect of it, but certainly the Wall comes from Hadrian's Wall, which
I saw while visiting Scotland. I stood on Hadrian's Wall and tried to
imagine what it would be like to be a Roman soldier sent here from
Italy or Antioch. To stand here, to gaze off into the distance, not
knowing what might emerge from the forest. Of course fantasy is the
stuff of bright colors and being larger than real life, so my Wall is
bigger and considerably longer and more magical. And, of course, what
lies beyond it has to be more than just Scots.
George maintains however that there is no any Character to Character correspondence between his characters and historical characters. From Citadel SSM 950:
The Wars of the Roses have always fascinated me, and certainly did
influence A SONG OF ICE AND FIRE, but there's really no one-for-one
character-for-character correspondence. I like to use history to
flavor my fantasy, to add texture and versimillitude, but simply
rewriting history with the names changed has no appeal for me. I
prefer to reimagine it all, and take it in new and unexpected
directions.
Other influences cited are Albigensian Crusades and A civil war labelled the Anarchy from English-Norman History for the prequel Princess and the Queen.
As for the question if GRRM was inspired by Byzantium, I can't find any mention of it from George's correspondence with fans or media. However, the point stands, there are many similarities to be seen between the two.