Hadrian's Wall

Well, the Romans certainly knew a thing or two about building forts; at the highest point for miles, where you could see everything in all directions (good for keeping a lookout) - but also where they were exposed to the elements (not quite so good for soft 21st century Brits who are not so fond of the wind and biting cold!). At least it stayed dry, even if it was rather windy.
Main photo is a section of wall near Birdowsald Roman Fort.
Extra (collage, top to bottom): view of the wall from Housesteads Fort; one of the  gates at Birdoswald; view of the section of wall in the main photo from Birdoswald.

Did I mention it was so, so cold!...