You’ll probably need to rephrase this question: A Song of Ice and Fire is not set in medieval times. It’s a work of fantasy. George RR Martin does borrow many elements from the Middle Ages (mostly European, but also Eastern), ranging from the more obvious (e.g. warfare, clothing, to a certain extent also architecture) to the very obscure (some minor details of Frankish and HRE history come to mind). Plus, of course, the entire plot premise is inspired by the English War of the Roses (even the names are similar – Lancaster and York).

It's fantasy, not historical fiction. It definitely draws plenty of inspiration from real life, but don't look to it for historical accuracy, because you won't find a whole lot.