Historical series are a challenge to write... always. How much history, how much fiction, how much emotion compared to action and battle scenes? How many love scenes if it's a romance...
It all depends on the specific genre. If you are writing true history with a gritty feel, if you know your audience, and they are all about the historical details, by all means give them a chuckful of them. If the focus of the story is romance or legends, however, or if it's a young adult story, other factors should be at the forefront at all times. Also consider the age and level of education of the reader. Will you have to explain every medieval word you use? Will they need a lexicon at the end of the book? You don't want them to be checking the dictionary at every page.
If that's the case, you may be in trouble. Because if you use fewer historical words and make sure they are illustrated in context, you should not need to explain anything.
When I started this series a few years back, after a decade of research and travels to Europe, I didn't really know how to define it, since it is based in authentic legends. It really didn't fit any mold. It was history, fantasy, legends, romantic, full of action... but with adult subjects. Publishers were interested, they liked the writing, but they didn't know what to do with it. But with the rejections came interesting comments. Once a NY agent told me there was a dichotomy between the modern narrative and the formal dialogue. So I had to unify the entire work. I simplified the dialogue, and formalized the narrative, so that the difference wouldn't jar the reader.
Finally, after many rewrites, I found the right tone for dialogue and narrative, the right amount of old words, just to give it the right flavor (historical fans like them). And I am proud to announce that Books We Love Ltd just releasee this first novel in the Curse of the Lost Isle series... PRINCESS OF BRETAGNE (here again I am taking a chance with a French word in the title, but it's all about taking chances).
You can find the book in kindle format HERE. Give it a thumbs up like if you feel like it.
Good luck in your own writing endeavors.
Swords, Medieval knights, Romance with a Kick