What are fens, anyway? They’re wetlands, like bogs or marshes. (If you want to be precise, fens are like bogs but less acidic.) Much of eastern England was composed of fens at the time of Anne’s childhood.
Back 8000 years ago, England was attached to the rest of Europe by a piece of land called Doggerland, which has disappeared, flooded and probably destroyed by a tsunami. Doggerland linked England and Holland. So when thinking about what England looked like before the fens were drained, think Holland. Parts of the east coast of England, like parts of Holland, are still below sea level.
Around the time of Anne of the Fens, Charles I hired a Dutchman named Cornelius Vermuyden to begin to drain the fens, starting in Lincolnshire north of where the Dudleys lived. The locals were opposed to the plan, mainly because Charles planned to fence in and take over a third of the recovered land. Cornelius was knighted in 1629 for his efforts, and went on later to drain a larger piece of the fens.
I spent some time in Lincolnshire before writing the novel, and I loved the fens. Much of them have of course been drained and filled in, but you can still see portions, with little creeks threading through them, that look like they must have looked 400 years ago. I loved their flatness, stretching on like ocean or fields of wheat in our midwest. I loved the birds, the many different kinds of waterfowl I had never dreamed of. And best I liked the little blue damselflies, like dragonflies only smaller, irridescent and sparkling in the sun.