What is the fine-grained topic of the following text: High ground is in short supply in Lincolnshire, so it’s no surprise that the steep hill that is today surmounted by Lincoln Cathedral was fortified early, firstly by the Celts, who called their settlement Lindon, “hillfort by the lake”, a reference to the pools formed by the River Witham in the marshy ground below. Only return the topic.
The history and geography of Lincoln Cathedral in Lincolnshire.