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

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.

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

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.

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

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

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

Tim Fitzharris / Fortress Mountain shrouded in clouds, reflected in lake, Kananaskis Country, Alberta, Canada