The massive defensive walls of the Kumbhalgarh Citadel, near Udaipur (India)
Kumbhalgarh is a fortress situated 82 km North-West of Udaipur.
Built on a hilltop 1100 metres above sea level, the fort of Kumbhalgarh has perimeter walls that extend 36 kilometres. The frontal walls are fifteen feet thick.
See this en.wikipedia.org/wiki/Kumbhalgarh.