(CNN) — An abandoned castle always cuts an impressive figure -- a blast from the past submerged behind ruined walls.
Says author Kieron Connolly, whose new book "Abandoned Castles" explores the world's most spectacular forgotten fortresses, they're also windows on long gone civilizations.
"Societies are like a body: centuries go by and the body decays," he tells CNN Travel.
"The castle's like the skull or like the teeth, in fact they even look a bit like teeth, sticking out, out of the ground. They give us some clues, some entries into the past."
Connolly's book features castles from around the world and across the ages -- including 19th century military forts in the French Alps, 13th century castles in the Scottish Highlands and a medieval fortress in Syria.
Questions with no answers
The book includes fortresses such as Fort de Malamot, built in 1889 in the Cottian Alps.
The pictures were sourced by Amber's Terry Forshaw.
Connolly traces his own interest in abandoned places back to his childhood.
"As a kid we would go on long walks and often it would be along a disused railway line," he recalls.
"These places had an air of mystery: 'Why's it closed down, where does it go, what does it look like when it's closed down, how quickly do branches grow through the tracks, stations start to crumble?'"
Layers of history
Spis Castle is a stunning Gothic-Romanesque hybrid.
Many of the castles in the book have histories spanning centuries.
Over the years different owners and different conflicts put their stamp on the castles' architecture.
"I love that you get this idea of layers of history," says Connolly. "You can see how a castle was built and then rebuilt and expanded, how the walls changed, how it passed back and forth and finally became obsolete."
Peeling back the layers offers a captivating peek into the past.
Take Spis Castle, in Slovakia. It started life as a Romanesque fort in the 12th century, before taking on a Gothic turn a century later.
In the 15th century, the castle was completely rebuilt. Later, new owners transformed it into a Renaissance family residence. It eventually burnt down in 1780, but remains a relic of these different architectural styles.
Genoese tower, Saint Florent, Corsica, France
Connolly's favorite story concerns Mortella Tower in Corsica.
"It was built in the 16th century, when Corsica was part of the Republic of Genoa, to defend against pirates," he explains.
"It was later blown up by the British during the Napoleonic Wars, but the structure and design of the tower impressed the British."
The design includes a circular tower with a flat roof, ideal for mounting artillery.
"They took that design and it became the Martello Tower, which is seen all across the British empire," Connolly says.
"So even though they destroyed the first one they ever found, they liked the idea of it and it became known all across the British empire. The fact that the spelling is different is apparently, I understand, simply a misspelling by the British."
Ireland's Ballycarbery Castle is slowing being absorbed by the natural world.
Other highlights in the book include Ballycarbery Castle in County Kerry, Ireland.
Situated on a dramatic location on the edge of the North Atlantic Ocean, all that remains of this 16th century fort are the stone walls.
The castle was damaged by English Civil War militarian Oliver Cromwell's troops, during his infamous Irish conquest.
Now this atmospheric fortress is being slowly absorbed into the natural landscape. The walls are clad in ivy and the first floor is covered in grass.
"It's an interesting idea, how nature reclaims these things," remarks Connolly.
Another striking spot is Golconda, near Hyderabad in central India.
Golconda Castle was the capital of the Qutb Shahi dynasty (from 1518-1687).
Built in the 16th century by the Qutb Shahi dynasty, the fort formerly housed the infamous Koh-i-Noor diamond -- later owned by Queen Victoria and now on display in Britain's Tower of London.
The citadel once included four individual forts, mosques, temples, royal apartments and gardens, now it's an arresting ruin.
Worlds gone by
Abandoned forts including Minard Castle, in Ireland, are a gateway to the past.
So why are photographers around the world so fascinated by ruins?
Connolly thinks it's because abandoned places are a meeting place for the past and the present.
"Abandoned places touch a nerve with people," Connolly says. "We're interested in worlds gone by, forgotten worlds."
All images taken from the book "Abandoned Castles" by Kieron Connolly (ISBN 9781782745228) published by Amber Books Ltd (www.amberbooks.co.uk) and available from bookshops and online booksellers now (UK RRP £19.99).