I agree with Sangahyando.
That doesn’t mean that good historical novels can’t have wild and improbable adventures, but they should at least be possible, and the history should be accurate.
IMHO a series that does this right is the Flashman books. Brilliantly funny, and with some improbable events… but every time some unusual character appears in the novels, and you check whether such a person actually existed, you will find he did, and was exactly as described. Every time the novels deal with historical events (which is often), you’ll find that all the facts are accurate.
For the uninitiated, the original Flashman was the villain in the novel Tom Brown’s School Days by Thomas Hughes (1857). Flashman is a bully, a coward, and a liar. In the end he is expelled from Rugby School, and the virtuous Tom Brown lives happily ever after.
In 1969 George MacDonald Fraser revived the character as an anti-hero in his own novels, and took up the story after Flashman’s expulsion.
Flashman tells the reader his own story in the first person, looking back at his life from the age of 80. He has been extremely successful (far more so than Tom Brown). He is now General Sir Henry Flashman, VC, KCB, KCIE, Chevalier of the Légion d’Honneur, U.S. Congressional Medal of Honor, San Serafino Order of Purity and Truth, 4th Class - and a member of the Board of Governors of Rugby School. He is extremely wealthy, a personal friend of the royal family, and esteemed by one and all as a perfect English gentleman.
However… in reality his character has remained unchanged from his schooldays with Tom Brown. He is, as he tells the reader, “a scoundrel, a liar, a cheat, a thief, a coward - and, oh yes, a toady.” He’s just been very good at it. The books are very, very funny - but are also unusually historically accurate.
Read the Look Inside on Amazon to get a taste of it.