Background knowledge From earliest times. the Channel served as a route for, and a barrier to. invaders to Britain from the Continent. It was crossed by Julius caesar's legion in 55 B C. and norman forces in 1066. However, when Napoleon and Hitler threatened to cross it, they failed. Therefore, the Channel has chiefly served as a physical barrier to the invasion of Britain. Transportation across the Channel used to be supplied by ferry boat service only.Background knowledge ▪ From earliest times, the Channel served as a route for, and a barrier to, invaders to Britain from the Continent. It was crossed by Julius Caesar’ s legion in 55 B.C. and Norman forces in 1066. However, when Napoleon and Hitler threatened to cross it, they failed. Therefore, the Channel has chiefly served as a physical barrier to the invasion of Britain. Transportation across the Channel used to be supplied by ferry boat service only