The "fields of corn" metaphor is not necessarily related to the crop known as maize. Corn means the main grain grown in an area in Britain. We call maize "sweet corn" in the UK to distinguish it from wheat, barley (e.g. "John barley-corn") or oats. It is not therefore an anachronism in itself.
The famous WILHELM SCREAM is heard at 31:55
When the Bishop [ Jonathan Rhys Meyers ] is praying he mentions that they will fall like corn before the blade. Corn didn't make it to Europe until 1492. This is an anachronism.