Reusing castles for modern invasion defence

Was thought given to reinforcing castles with modern defenses when invasion threatened?

German and British forces both adapted old fortifications, including concrete work in Jersey and surviving modern layers at Dover and Pendennis castles.

Historic castles continued to offer useful terrain, walls, underground space, and commanding positions when modern invasion threatened. German forces added extensive concrete defenses around castles in Jersey. British planners likewise reused and altered older fortified sites rather than treating them as obsolete monuments. Dover is the most famous example, while Pendennis at Falmouth preserves an especially interesting mixture of defensive periods. These sites form layered military landscapes in which each generation adapted the position to new weapons and threats.