https://www.amazon.co.uk/KNIGHTMARE-Beverley-Cann-ebook/dp/B08TCGFZQP/

It's got nothing to do with any TV series you may be thinking about - although it is about someone from the modern world going back in time by mysterious means to the 12th Century and spending a lot of time in an English Castle in which people die frequently and in rather gruesome ways - but it is rather good....