Castles have been built in Cornwall since early man fortified many points around the coast. Many "follies" also exist where just a wall was built in the hope that an invading fleet would imagine there to be fortifications and weaponry hiding behind it.
There are two Tudor castles at St Mawes and Pendennis Castle in Falmouth that guard the Carrick Roads. The thought of invasion hung heavily over Cornwall for the last five hundred years. There were the odd skirmishes with the Spanish or French, and many headlands were used to house beacons as warning of approaching enemy fleets.
Even in this century gun emplacements and coast lookouts sprouted everywhere. Most of these were not too permanent, and all that remains is the concrete bases on many headlands and vantage points.