Fortress Guernsey?

We are walking and motoring on a short break with friends. We think its a lovely island .

I cannot recall seeing so many defences per mile of coast anywhere I have ever been. 

The island is like a history of forts, castles, towers and walls from circa 13th to 20th century and possible before. Surely the Romans where here?

 Anyway I think this is Fort Le Marchant for the very curious. 

And the extra is a tower similar to a Martello tower. They appear to have been built within sight of each other around the island's  coast which has an abundance of lovely sandy beaches,