We are accustomed to think of England in terms of Shakespeare's 'precious stone set in a silver sea', safe behind its watery ramparts with its naval strength resisting all invaders. To the English of an earlier period - from the 8th to the 11th centuries - such a notion would have seemed ridiculous. The sea, rather than being a defensive wall, was a highway by which successive waves of invaders arrived, bringing destruction and fear in their wake.