The British Isles have, nevertheless, been invaded successfully several times, notably by the Romans, Saxons, Vikings and the Norman French. Prior to the Union of the Crowns in 1603, England and Scotland had to be concerned with the threat of invasions across the border and for England there were frequent fears that an attack from Scotland might be accompanied by an invasion, by a continental power, from the sea. It is generally true, however, that since the Norman Conquest, the British Isles have never been successfully invaded. William of Orange's landing at Torbay can be cited to the contrary but this was essentially part of the internal revolt that we know as the 'Glorious Revolution'. The Act of Union of 1707, consolidating the Union of the Crowns, created a Britain that was stronger than its constituent parts in the face of possible invasion.

Britain's first lines of defence have always been the sea itself, with a bit of help from the weather, and many miles of rocky coastline. Without command of the seas, such natural defences were never sufficient to withstand a determined attempt at invasion and the main protector of Britain has been, since the reign of Henry VIII, the Royal Navy. During the late Saxon period, kings controlled naval forces but the structure of feudal government and the limitations on central taxation meant that until the sixteenth century there was little in the way of a permanent fleet to safeguard England from invasion; safety lay more in the absence of neighbouring powers with significant naval forces. Since the sixteenth century, the Royal Navy and its ability to command the seas around Britain has been essential to security.

There has always been, however, the need for further lines of defence in the shape of coastal fortifications and the coastline of Britain is marked by remains of successive generations of coastal defences. Only a centralised administration able to raise an income from taxation is capable of organising a national defence system, so it is not surprising that it was when much of Britain was part of the Roman Empire that we find the earliest evidence for a formidable coastal defence system. The Romans built milecastles, forts, watch-towers and signalling stations along many miles of coastline. The coast of north-west England was considered particularly vulnerable and there was a major fort at Maryport and a chain of watch-towers extended down the Cumbrian coast from Bowness to St Bees Head. Along the "Saxon" shore, ten forts were built between Hampshire and Norfolk.

Saxon and then medieval English kings were conscious of the need to defend Britain's coasts but for the most part exercised less direct control over coastal defences. The castle is best described as a fortified private residence. Until the strategic castles built by late medieval monarchs, defence was, to a considerable degree, the responsibility of local feudal magnates or of individual ports.

From the Tudor period we find coastal defence systems that highlighted the defence of commercial ports of importance, points where landings might be attempted and strategic naval ports. The defence of the capital was obviously important and the approaches to the Thames were heavily defended. Besides building a strong navy, Henry VIII created a great chain of coastal fortresses in the 1540s that assisted England in surviving the threat from Spain in the later sixteenth century; Southsea Castle, built to protect Portsmouth, is one of the best surviving examples.

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