DNF out of sheer rage.
Everyone who said this was well-researched is lying, or are themselves not well-informed enough of the history to tell.
The maps were real. That automatically sets up a high expectation for me. No matter how much you say "I used real life only as inspiration for fiction", I will hold every error against you.
The TL;DR is I feel like I could've made the same story 10 years ago, armed with barely functional Wikipedia knowledge of Sengoku Japan and Google Translate. Everything is so clichéd, and brings absolutely nothing new to the ninja and samurai trope. It's only pseudo-unique if you compare it to vampires, werewolves, dragons, and every other generic Tolkien knockoff on the market. And if these wonky depiction is purposeful rather than genuine errors, then it's even more asinine. What's the point of researching, if you then end up chickening out and running with the dead-tired overused tropes?
I cringed at the Glossary right off the bat. No, there is no such thing as Bushidō in 1500s Japan. Warriors may follow some personal set of honour code, but it will be just a generic virtue or some such. The word Bushidō, as a term, does not exist. The blubber about jōnin and chūnin ninjas, and all the smoke bombs and such are utter garbage. Ancient garbage written in the 1600s or 1700s it may be, but still garbage all the same.
And then, the clincher: Will people just bloody STOP saying Nobunaga wants to destroy Iga? I don't care if you ascribe semi-noble reasoning for his quest. Perhaps had things been left alone, the conquest of Iga is an eventual inevitability. However, the fact remains that in reality Nobukatsu initially set off to attack Iga without consulting his father.
So putting Nobukatsu there on-scene, but then still insisting on saying "Nobunaga ordered it though" is such a tell just how little research on the matter the author did. Just based on even the most cursory of Wikipedia research, 8 years ago I already knew Nobukatsu was the instigator of the Iga war, and finding out why was the whole point of me learning Japanese history. And they had LESS info out online back then than they do now.
Nobunaga himself fighting— hah! Nobunaga wasn't even there to supervise. Both times. He just came in at the end of the second invasion — well after everyone cleaned up the place and no more fighting is to be done, mind you — to inspect the land.
And then if you dig down further, you actually have claims that Iga insiders was the one who came knocking at Oda's door asking them to please invade Iga. Was it a trap to wreak havoc among the Oda ranks? Maybe, but on surface reading all it looks like is a whistle-blower despised their own province so much they wanted it gone.
Also, how lazy do you have to be that you don't even know the names of Nobunaga's vassals during the Iga invasion? The Nobunaga scene involved some NPC Bob sort of character called Ito. Wikipedia has a list of people's names. Can't you even namedrop the ever-famous Mitsuhide? Hideyoshi? Not Yasuke? I'm willing to forgive the naming of wrong vassals since even Japanese people themselves do it (again, WIKIPEDIA tells you which persons are actually there, why are you naming bullshit names???). Like, I know Mr Mortensen knows about Hideyoshi and Mitsuhide since the book goes all the way up to Honnouji.
So yeah... boring ninja tropes. It's only good and unique and interesting if you know jack doodle squat about real Japanese history, and only consume Western-based medieval stories.