Once again, thanks, Jason. This is all so interesting!

It was about this period that Martello Towers started to go up all along the southern coast as a defence against a French invasion, so the Austens and others who visited the south coast would have been familiar with the sight and unable to forget the invasion threat completely.

Here's a link to a BBC history site about them. Many of them are still around, and they may be some of the most durable buildings of the period.