Coastal defenses work by changing the attacker's calculation

Pendennis and St Mawes were artillery forts guarding the Carrick Roads anchorage at Falmouth. The port was valuable as a fleet base even though Cornwall was an unattractive route for a wider invasion. Successive governments therefore maintained and modernized the defenses so that a raid or assault would cost more than its likely reward. A fortification can succeed without fighting if it diverts enemy resources, delays action, or deters the operation altogether. Local maintenance also had a less official incentive: keeping the defenses in good order reduced central-government attention and made smuggling easier.