The iconic appearance of Edinburgh Castle today is largely the result of the threat posed to the government by the Jacobites. Such was their fear of the 'King over the Water' that they initiated a massive program of re-fortification; modernising the defences of Edinburgh, Stirling and Dumbarton Castles along with a series of dedicated forts and barracks such as at Fort William, Fort Augustus and Inverness.
The sagacity of this policy was vindicated when Edinburgh and Stirling Castles held out throughout the '45 in spite of being attacked, but paradoxically Fort Augustus, the 'Jewel in the Crown', literally landed in the Jacobites' lap when a mortar shell denoted its powder magazine sending it sky high thanks to the skill of a French gunner.
What these forts had in common were bastioned defences equipped with the ubiquitous sentinel house. The latter allowed the patrolling sentry some protection from the elements and provided a sight line along the walls. Some were utilitarian, others embellished, bringing a little light relief to otherwise bland fortifications.
Sentinels are not confined to Scotland for wherever the European went - Africa, India or the Americas - they left as their legacy the bastioned fort and the sentinel house.
Here are just a few examples of this truly universal feature: