So what's so special about this island?

I was walking last evening, near where we're renting a house on Jersey. We were struck by the sheer volume of fortifications in the area.



From right to left, a 13th century castle, a WWII gun emplacement and a Napoleonic War Martello Tower (turn the other way and you'll see five more Martellos within a couple of miles). Such a stunning amount of investment of effort for defending an island a few miles wide...