This ditch , being the western defensive ditch,of a uni-vallate, pre-Roman British tribal hillfort ,believed to have been the home of a tribe called the Trinovantes, is called the Devil's Dyke,because our Saxon forebears thought it too massive for mere humans to construct, thus it had to be the Devil's work.Hence the title.It is also believed by many Romanophiles to be where Julius Caesar took the surrender of the Britons in BC 54.
see http://www.theaa.com/walks/wheathampstead-where-julius-caesar-marched-420609
see http://en.wikipedia.org/wiki/Caesar's_invasions_of_Britain
see http://en.wikipedia.org/wiki/Devil%27s_Dyke,_Hertfordshire