(CNN) — Wandering around the grounds of Hikone Castle by Lake Biwa, in central Japan , the scene looks like something straight out of a postcard.

Known as the "Water Castle," the complex features towering stone walls, romantic sakura blossoms, trick doors to confuse enemies, and steep stairways leading up to wooden keeps where travelers stop to admire panoramic views of the lake below.

But there's much more to this story than just pretty scenery.

In her new book "Samurai Castles," author Dr. Jennifer Mitchelhill and photographer David Green spent over three years documenting 24 of Japan's samurai-era castles.

The idea? To understand the aesthetic details, cultural values and historic happenings surrounding these architectural marvels.

An architectural historian at the University of Melbourne, Australia, Mitchelhill has bounced in and out of Japan for the past 30 years for research projects and teaching jobs.

"One of these research projects was a documentation of the reconstruction of Kanazawa Castle," recalls Mitchelhill.

"The lack of accurate information on Japanese castles in English became apparent, so I decided to visit, photograph and document as many castles as I could over the next three years."

This led to her first book, "Castles of the Samurai: Power and Beauty," also with Green, in 2003. For the next 15 years, during postgraduate studies, Mitchelhill focused on Japanese design.

Here, the scholar reveals everything you ever wanted to know about the ancient samurai castles of Japan so you can plan your own visit.

What inspired you to work on this book?

Kumamoto Castle's walls are said to be impossible to scale. David Green

"My initial interest in Japanese castles was the architecture. I was intrigued as to how a military structure could be strong and impenetrable, yet graceful and intricately detailed at the same time," says Mitchelhill.

"Consequently, the emphasis of our first book was on the architectural details. However, the individual stories behind each castle still needed to be told: "How they came to be, why they are situated where they are, and why a particular lord built the castle, yet did not stay there very long."

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What's important to understand about the samurai?

"The samurai were warriors. The period in which the castles in this book were built spanned the end of the Warring States period in the late 16th century until the early years of the Tokugawa Shogunate or Edo era (1600-1868)," says Mitchelhill.

"This era was a feudal period with a strict class system. The Shogun was the supreme military leader, the daimyo was lord of a region, and the samurai were the daimyo's retainers or army.

"But the samurai were not just warriors. They were also castle-builders, politicians and local leaders with a great aesthetic sense."

How many castles are there in total in Japan?

Mitchelhill's book documents 24 Samurai-era castles, including Iyo-Matsuyama, on Shikoku island, pictured here. David Green

"The number of castles has varied greatly over the past 500 years," says Mitchelhill. "During the Warring States period in the 15th and early 16th centuries, thousands of fortifications were built to protect a warrior's territory."

"Toward the end of this period as Japan was being unified by three powerful daimyo, hundreds of larger, stronger castles with stone walls were built.

"Most of these were ordered to be demolished, however, when the Tokugawa achieved absolute power in 1615, leaving about 180 'modern' castles (built between 1575 and 1620) as featured in this book."

What's happened since then?

"Many castle buildings have been reconstructed, initially in concrete," says Mitchelhill. "In the last 20 years, there has been a shift to reconstructing in traditional materials using traditional building techniques."

"The castles in this book are the most intact or important castles that remain today. However, it is possible to still find the remains of hundreds of castles throughout Japan."

Who originally designed these castles?

A closer look at Nagoya Castle's Kiyomasa stone. David Green

"Castles were designed by samurai or their lords," says Mitchelhill. "Two of the most well-known castle designers were the daimyo Todo Takatora and Kato Kiyomasa."

"Todo Takatora is said to have designed about 20 castles, the largest and most important being Edo castle.

"Other castles by Todo were Wakayama, Uwajima, Imabari, Iga Ueno and Sasayama."

"Kato Kiyomasa designed Kumamoto and Nagoya castle. He was known for his skill at building sheer stone walls. There is a huge stone in one of the walls at Nagoya castle dedicated to him called the Kiyomasa stone."

What did you find surprising when documenting the architecture?

"Even though samurai castles are architecturally similar in terms of stone walls, moats and timber towers, they differ vastly in size, layout and specific aesthetic features," says Mitchelhill.

"Each castle had to be unique for defensive purposes in order to confuse an invading enemy. In addition, the enormous castles were built in just a couple of years using local manual labor.

"On the orders of the daimyo, peasants built stone walls of up to 30 meters high (98 feet), dug moats up to eight meters (26 feet) deep, transported huge boulders to be placed at important entrance ways, and constructed countless timber, mud and plaster buildings."

Which castles have the most interesting defensive strategies?

"Nagoya, Osaka and Kumamoto castles all have massive sheer stone walls which curve inwards.

"The stones in these walls are fit tightly together to eliminate footholds for intruders seeking to scale them," explains Mitchelhill.

"Osaka's walls rising from the moats resemble an open fan, with each crease or corner protected by a tower. Castle defenders could shoot at invaders through holes and stone dropping windows in these towers.

"Meanwhile, Kumamoto castle's walls are nicknamed nezumi-ishi (mouse walls) as they were deemed impossible for even a mouse to scale," says Mitchelhill.

"Unfortunately, the 2016 earthquake destabilized many of the walls and buildings at Kumamoto castle. They are expected to take many years to repair."

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Of all castles you've visited, which one is the most beautiful?

Built in 1603, Nijo Castles was the residence of the Edo era's first shogun David Green

"The palace at Nijo castle, in Kyoto, is extremely beautiful. It is the oldest surviving castle palace at over 400 years old," says Mitchelhill.

"Built by Tokugawa Ieyasu (founder and shogun of the Tokugawa shogunate) in the Edo era 1600-1868, Ieyasu employed the most skilled artisans of the day to decorate the palace walls and screens with exquisite paintings and carvings.

"Through this opulence, Ieyasu was able to impress his power upon visitors to the palace."

Did you encounter any surprising stories along the way?

"Maruoka castle has a 'human post' (called a hitobashira) entombed in its stone base.

"During the castle's construction in 1576 there were problems stabilizing the stonewalls. It was decided that a human sacrifice was needed to appease the gods," says Mitchelhill.

"A blind peasant woman called Oshizu volunteered to be a hitobashira in exchange for her son to be taken in by the lord of the castle and made a samurai.

"Unfortunately, the lord was unable to fulfill his promise as he was transferred to another castle soon after.

"Whenever the moat floods, as it often used to, it was said to be the tears of Oshizu."

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Which castles is considered the most famous?

The bright white Himeji castle dates to Japan's feudal period. David Green

"Himeji castle is the most famous castle in Japan today. It is the largest original extant castle with enormous grounds, a huge tenshu (the main tower) and a number of original outer structures and gates," says Mitchelhill.

"It is beautiful, imposing and easily accessible by train from Osaka and Kyoto (in the Hyōgo Prefecture)."

And an overlooked gem you'd recommend?

"Kochi castle is one of a number of excellent castles on the island of Shikoku (southwest of Kyoto) that are worth visiting along with Marugame, Iyo-Matsuyama and Uwajima castles," says Mitchelhill.

"These castles make up four of the 12 remaining castles in Japan with an original tenshu.

"Kochi castle has a number of interesting features: The arrangement of buildings within the innermost enclosure is unique, with the lord's reception rooms adjacent to the main tower. These buildings were usually located in a lower or outer compound.

"Slatted windows in the low mud-plastered walls surrounding the inner compound enabled samurai to keep watch; iron spikes line the top of the stone base of the tenshu to deter would-be invaders; and a 'trick gate' was used to trap an invading army."

Which is the most remote?

Hagi is one of Japan's most remote castles. David Green

"Hagi castle, on the southwest coast of Japan, is one of the more remote castles."

"It is not well known, and difficult to get to requiring various forms of public transport. However, it is definitely worth visiting," says Mitchelhill.

"The samurai districts surrounding the inner castle enclosures are very well preserved and give insight into an Edo-era castle town. Hagi castle itself has an interesting layout with a fortress lookout on top of Mount Shizuki and a tenshu surrounded by a moat at the foot of the mountain."

"The mountain is on a peninsula which juts into the sea. It is an excellent example of the use of topography to create an impenetrable fortress."

Any story behind it?

"Yes, it's fascinating: Mori Terumoto was one of the richest and most powerful daimyo in Japan.

"But after supporting the losing side at the Battle of Sekigahara in 1600, he was stripped of his land at Hiroshima and banished to the remote location of Hagi," says Mitchelhill.

"The Mori never quite got over their banishment and this area became a hotbed for dissent against the Tokugawa Shogunate. It was from Hagi that many of the instigators of the Tokugawa demise came from in the 1860s."

Of all the castles, do you have a personal favorite?

Hikone Castle was built in 1603. David Green

"One of my favorite castles is Hikone which has a number of unique, original buildings within its extensive grounds," says Mitchelhill. "There is a stunning stroll garden with a lake, tea house and a small palace-style residence at the foot of the hill where Hikone's main tower stands."

"The main tower is small, but has beautiful proportions, exquisite black and gold decoration, black timber bell-shaped and square push-out windows and a mix of roof-gable styles.