Intersting stuff. I started by looking of Offa’s wall on wikipedia and kept followoing fascinating refrences one after the other. The death of King Penda, the birth of Northumbria, the Venerable Bede etc, etc.
That was the problem with encyclopedias when I was a lad — go to look something up, flip open to find it, see something else, read it, look up the related articles, forget the original question. ;’)