A slighting is the deliberate destruction, partial or complete, of a fortification
Fortifications are military constructions and buildings designed for defence in warfare and military bases. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs...

 without opposition. During the English Civil War this was to render it unusable as a fort.

Middle Ages

In England during the Middle Ages adulterine (unauthorised) castles if captured by the King would usually be slighted.

English Civil War

During the English Civil War
English Civil War
The English Civil War was a series of armed conflicts and political machinations between Parliamentarians and Royalists...

many castles and fortified houses were slighted by the Parliamentarians to stop them being used by the Royalists. Most of the destruction was in Wales, the Midlands, and Yorkshire. The coastal fortifications were spared by the Commonwealth as they might have been useful for hindering a Royalist or foreign invasion.
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