Guy Edwardes / Dawn mist around Hambledon Hill from Bullbarrow Hill, Cranbourne Chase, Dorset, England, UK. February 2016.

Gavin Hellier / Timelapse from day to night looking towards the Peter and Paul Fortress, St, Petersburg, Russia, May 2016. Hellier

Tim Fitzharris / Fortress Mountain, Kannaskis Country, Alberta, Canada

Kirkendall-Spring / Castle / Chateau d'Angers fortress and gardens, Angers, France, Europe

Christophe Courteau / Stars Trails around the North Star, above sea and National Fort, Saint-Malo, Brittany, France.

Yashpal Rathore / Bengal Tiger (Panthera tigris), walking through ancient arch of fort, Ranthambore National Park, India

Juan Carlos Munoz / Dun Aonghasa / Fort of Aongus hill fort. Inishmore, Aran Islands, County Galway, Republic of Ireland. May 2011.

Guy Edwardes / Harlech Castle, Gwynedd, Wales, UK, August 2005.

Guy Edwardes / Harlech Castle, Gwynedd, Wales, UK, December 2008.

Ashley Cooper / La Calahorra Castle in La Calahorra at the foot of the Sierra Nevada mountains in Andalucia, Spain,

Ashley Cooper / La Calahorra Castle in La Calahorra with spring flowers, at the foot of the Sierra Nevada mountains in Andalucia, Spain.

Eladio Fernandez / Guide leading visitor's horse up to La Citadelle fortress, UNESCO World Heritage Site, Haiti. August 2016.

Eladio Fernandez / Row of cannons in La Citadelle fortress, UNESCO World Heritage Site, Haiti. August 2016.

Eladio Fernandez / View from one of the fortresses windows into the backyard where all the cannon and mortar balls are stored. La Citadelle, UNESCO World Heritage Site, Haiti, August 2016.

Ashley Cooper / 12th century grain store or Agadir at the Berber village of Tizgui, Anti Atlas mountains, Morocco, North Africa. April. 2012.

Axel Gomille / Ranthambhore Fort with resthouse Jogi Mahal below, Ranthambhore National Park, Rajasthan, India

Alan Williams / Dolwyddelan Castle, mountain fortress remains, near Dolwyddelan, Snowdonia National Park, North Wales, UK. December 2017.

Nick Upton / Ancient stone staircase with 913 steps zigzagging up to Palamidi Castle, a Venetian fortress on a hill overlooking Nafplio and the Argolic Gulf, Argolis, Peloponnese, Greece, July.

Nick Upton / Palamidi Castle, a Venetian fortress high on a hill overlooking Nafplio, the first capital of modern Greece, Argolis, Peloponnese, Greece, July 2017.

Nick Upton / Palamidi Castle, a Venetian fortress high on a hill overlooking Nafplio and the Argolic Gulf, Argolis, Peloponnese, Greece, July 2017.

Nick Upton / Harbour entrance with Bourtzi Island, a Venetian fortress once used as a prison, in the background, Nafplio, Argolis, Peloponnese, Greece, August 2017.

Will Watson / British Camp Iron-Age hill fort on the Herefordshire Beacon, Malvern Hills AONB, Herefordshire, viewed from Worcestershire, England, UK, August 2017.

Will Watson / Southern extension of British Camp Iron-Age hill fort on the Malvern Hills AONB, 2nd Century BC, Herefordshire, England, UK, August 2017.

Philippe Clement / Tourists looking at Dunnottar Castle, ruined medieval fortress near Stonehaven on cliff along the North Sea coast, Aberdeenshire, Scotland, UK, May 2017

Philippe Clement / Dark clouds above Dunnottar Castle, ruined medieval fortress near Stonehaven on cliff along the North Sea coast, Aberdeenshire, Scotland, UK, May 2017

Loic Poidevin / Fort Enet and Ile d'Aix, Charente-Maritime, France, Atlantic Coast. July 2017.

Loic Poidevin / Aerial view of Fort Liedot, Ile-d'Aix, Charente-Maritime, France, July 2017.

Loic Poidevin / Aerial view of Fort Enet near Ile d'Aix and Fort Boyard, Charente-Maritime, France, July 2017.

Tim Fitzharris / Fortress Mountain from Wedge Pond, Kananaskis Country, Alberta, Canada

Mark MacEwen / Mehrangarh Fort, located in Jodhpur, Rajasthan, India.

Mark MacEwen / Mehrangarh Fort, located in Jodhpur, Rajasthan, India. March 2015

Mark MacEwen / Hanuman Langurs (Semnopithecus entellus) on walls, Mandore Garden, Jodhpur, India. March 2015

Angelo Gandolfi / Ancient fortress and castle of 'Le Castella' Crotone, Calabria, Italy, June 2013.

Ingo Arndt / Looking down at Tiger Fort with water reserve, Jaipur, Rajasthan, India

Ingo Arndt / Meherangarh Fort overlooking city, Jodhpur, Rajasthan, India

Ingo Arndt / Red Fort, fine example of Mughal architecture, Delhi, India

Ingo Arndt / Jaisalmer Fort overlooking city landscape, on edge of Thar Desert, Rajasthan, India

Philippe Clement / Waves crashing against Vauban's Fort Mahon at Ambleteuse during winter storm along the North Sea coast, Côte d'Opale / Opal Coast, France. January 2017

David Noton / Ramparts of the prehistoric hill fort on Hambledon Hill above the Blackmore Vale, Dorset, England, UK, September 2015.

David Noton / Prehistoric hill fort of Hambledon Hill above the Blackmore Vale, Dorset, England, UK, September 2015.

Martin Gabriel / Bahla Fortress, the oldest fortress of Oman (13th century), UNESCO World Heritage Site, Sultanate of Oman

Martin Gabriel / Bahla Fortress, the oldest fortress of Oman (13th century), UNESCO World Heritage Site, Sultanate of Oman

Martin Gabriel / European Oystercatchers (Haematopus ostralegus) and gulls at the beach in front of As Sawadi Island with As Sawadi Fort, Sultanate of Oman, January.

Tim Laman / Fort Tolukko, restored Portugese fort dating from 1512 overlooking Ternate Harbor. Maluku Islands / Moluccas, Indonesia, July 2008

Tim Laman / Fort Tolukko, restored Portugese fort dating from 1512 overlooking Ternate harbor. Maluku Islands / Moluccas, Indonesia, July 2008

Peter Lewis / Sunset over Hambledon Hill Iron age Hill fort, Dorset, England, UK. April 2014.

Juan Carlos Munoz / Staigue Fort, Castlecove, Iveragh Peninsula, County Kerry, Republic of Ireland. September 2015.

Juan Carlos Munoz / Staigue Fort, Castlecove, Iveragh Peninsula, County Kerry, Republic of Ireland. September 2015.

Juan Carlos Munoz / Staigue Fort, Castlecove, Iveragh Peninsula, County Kerry, Republic of Ireland. September 2015.

Juan Carlos Munoz / Staigue Fort, Castlecove, Ring of Kerry Trail, Iveragh Peninsula, County Kerry, Republic of Ireland. September 2015.

Pete Oxford / Zeelandia, a Dutch fort built 1743, Fort Island on Essequibo river, Guyana, South America

Nick Turner / Uley Bury Iron Age Hill Fort, Uley, Gloucestershire, UK. May 2016.

Nick Turner / Uley Bury iron-age hill fort on Cotswold escarpment, Gloucestershire, UK. Aerial drone with CAA permit. November 2015.

Nick Turner / Uley Bury iron-age hill fort on Cotswold escarpment, defensive ditches still visible. Gloucestershire, UK. Aerial drone with CAA permit. November 2015.

Nick Turner / Uley Bury iron-age hill fort on Cotswold escarpment, defensive ditches still visible. Gloucestershire, UK. Aerial drone with CAA permit. November 2015.

David Pattyn / Male peacock (Pavo cristatus) sitting on fort wall, Ranthambore National Park, Rajasthan, India

Graham Eaton / Iron age hut circle, within the Tre'r Ceiri Hill Fort, above Nefyn, Gwynedd, Wales, September 2013.

Graham Eaton / Hut circle groups within the Iron Age hill fort, Tre'r Ceiri near Yr Eifl, above Nefyn, Gwynedd, Wales, September 2013.

Graham Eaton / Hut circle groups within the Iron Age hill fort, Tre'r Ceiri near Yr Eifl, above Nefyn, Gwynedd, Wales, September 2013.

Constantinos Petrinos / Fasilides Castle, ruined fortress city founded in the 17th centurary. Amhara National Regional State, North Gondar Administrative Zone, Ethiopia, Africa, March 2009

Constantinos Petrinos / Fasilides Castle, ruined fortress city founded in the 17th centurary. Amhara National Regional State, North Gondar Administrative Zone, Ethiopia, Africa, March 2009

Constantinos Petrinos / Fasilides Castle, ruined fortress city founded in the 17th century. Amhara National Regional State, North Gondar Administrative Zone, Ethiopia, Africa, March 2009

Christophe Courteau / Herring gull (Larus argentatus) on Saint-Malo ramparts, in front of Fort National. Saint-Malo, Ille-et-Vilaine, France, March 2014.

Axel Gomille / Panoramic of the city of Jaisalmer, nicknamed the Golden City, view of the fort from Gadisar Lake, Thar Desert, Rajasthan, India. February 2012.

Steve O. Taylor (GHF) / Ancient abandoned fort citadel at Segedine , northern Niger, 2005.

Steve O. Taylor (GHF) / Abandoned ancient fort at Segedine , northern Niger, 2005.

Steve O. Taylor (GHF) / Ancient abandoned fort that once guarded trade routes. Northern Niger, 2005.

Steve O. Taylor (GHF) / Ancient fort and oasis at Segedine on the desert trail to southern Libya. Niger, 2005.

Steve O. Taylor (GHF) / Ancient fort at Segedine on the desert trail to southern Libya. Niger, 2005.

Steve O. Taylor (GHF) / Stone wall inside the ruins of Bunce Island slave trading fort. Sierra Leone, 2004-2005.

Angelo Gandolfi / Florianopolis, the bridge and fortress. Santa Catarina State, Brazil, September 2010.

RHONDA KLEVANSKY / Canon lighting ceremony at sunset, Fort, Santiago de Cuba, Cuba, November 2011.

Rhonda Klevansky / Four people in traditional costumes lined up against wall, part of a Canon lighting ceremony at sunset, Fort, Santiago de Cuba, Cuba, November 2011.

Colin Varndell / Ramparts on west end of Hambledon Hill prehistoric hill fort, Dorset, UK. June 2013

Colin Varndell / West entrace to Maiden Castle iron age hillfort near Dorchester, Dorset, UK. June 2013

Franco Banfi / Castello / castle Aragonese, on island off Ishcia, Italy, Tyrrhenian Sea, Mediterranean

Franco Banfi / Castello / castle Aragonese, Ischia Island, Italy, Tyrrhenian Sea, Mediterranean

Andrew Parkinson / The impressive fortress island of Boreray in the World Heritage Site of St Kilda. St Kilda, Outer Hebrides, Scotland, UK. May.

Ben Lascelles / Tsaparang (sometimes known as the mythical Shangri-la) was the capital of the ancient kingdom of Guge in the Garuda Valley, Ngari Prefecture, Western Tibet. Tsaparang is a huge fortress perched on a p...

Ben Lascelles / Looking out from inside cave in Tsaparang (sometimes known as the mythical Shangri-la) was the capital of the ancient kingdom of Guge in the Garuda Valley, Ngari Prefecture, Western Tibet. Tsaparang i...

Ben Lascelles / Buddhist prayer flags outside window frame at Tsaparang (sometimes known as the mythical Shangri-la) was the capital of the ancient kingdom of Guge in the Garuda Valley, Ngari Prefecture, Western Tibe...

Ben Lascelles / Doorway in Tsaparang (sometimes known as the mythical Shangri-la) was the capital of the ancient kingdom of Guge in the Garuda Valley, Ngari Prefecture, Western Tibet. Tsaparang is a huge fortress per...

Gavin Hellier / Brimstone Hill Fortress which is situated 790 feet above sea level, the 18th Century compound is lined with 24 cannons and is the largest and best preserved Fortress in the Caribbean - Brimstone Hill...

Gavin Hellier / Elevated view of Brimstone Hill Fortress which is situated 790 feet above sea level, the Brimstone Hill Fortress National Park is a UNESCO World Heritage Site, St Kitts, St Kitts and Nevis, Leeward Is...

Gavin Hellier / Elevated view of Brimstone Hill Fortress which is situated 790 feet above sea level looking towards St. Eustatius Island and the Caribbean Sea, the Brimstone Hill Fortress National Park is a UNESCO Wo...

Gavin Hellier / Brimstone Hill Fortress which is situated 790 feet above sea level, the 18th Century compound is lined with 24 cannons and is the largest and best preserved Fortress in the Caribbean, a National Park...

Gavin Hellier / Large cruise liner docked in the harbour next to the Old Town, Fjord and mountains from the Walls of the Kotor Fortress which forms a continuous belt around the Old Town of Kotor, Bay of Kotorska, Adr...

Gavin Hellier / Large cruise liner docked in the harbour next to the Old Town, Fjord and mountains from the Walls of the Kotor Fortress which forms a continuous belt around the Old Town of Kotor, Bay of Kotorska, Adr...

Philippe Clement / Vauban fortress at the Pointe des Capucins at Roscanvel, Finistère, Brittany, France, June

Philippe Clement / Vauban fortress at the Pointe des Capucins at Roscanvel, Finistère, Brittany, France, June

Philippe Clement / Vauban's Fort Mahon on the beach at Ambleteuse, Côte d'Opale, France, March

Benoit Stichelbaut / Fort on Ile Cigogne in the Glenan Archipelago, Finistere, France

Benoit Stichelbaut / Fort Cigogne on the Glenan Archipelago, Finistere, France

Benoit Stichelbaut / Fort la Latte and Cape Frehel, Cotes d'Armor, France

Luiz Claudio Marigo / Castillo de San Felipe de Barajas (San Felipe de Barajas Fortress), the largest military construction of Spanish colonial times in South America in Cartagena de Indias, a UNESCO World Heritage City. B...

Jeff Vanuga / Fortress Geyser in the Firehole River Group. Lower Geyser Basin, Yellowstone National Park, Wyoming, September 2010.