We often 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 eighth to the eleventh centuries - such a notion would have seemed absurd. To them, the sea was not a defensive wall, but a highway by which successive waves of invaders arrived, bringing destruction and fear in their wake. Drawing from a wide range of sources, England's Medieval Navy, 1066-1509 looks at the ways in which English kings after the Norman Conquest learned to use the Navy of England - a term which at this time included all vessels, Royal or private, no matter what their purpose - to increase the safety and prosperity of the kingdom. Within this narrative, Susan Rose describes the design and building of ships and harbour facilities, the development of navigation and ship handling, as well as the world of the English seaman. Comparisons with the navies of England's closest neighbours, particularly France and Scotland, are also made, while notable battles such as Damme, Dover, Sluys, and La Rochelle are included to explain the development of tactics and arms during the period. A lucid and illuminating account, England's Medieval Navy, 1066-1509, shows how the unspoken aim of successive monarchs was to begin building ''the wall'' of England - its naval defenses - with a success that would become apparent in later centuries.