Defensible frontiers are crucial to a country's security as, for instance, Poland knows to her cost. Mountain ranges come in useful but it is better still to be, like Britain, an island and have coastlines rather than land frontiers to defend.
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.
During the Anglo-Dutch wars of the 1650s and 1660s Britain was not always able to defend its shores successfully, and in 1667 the Dutch ships arrived at the Medway, troops were landed and three of Britain's largest ships were burnt as they lay at anchor. In response, Tilbury Fort was built to prevent another such attack on the Thames estuary; Portsmouth became an enclosed fortified town and, at Plymouth, an imposing citadel was erected on Plymouth Hoe.
Even the rise of Britain to become the world's foremost naval power in the eighteenth century did not remove the need for coastal defences. Despite the defensive installations created by the state, isolated communities needed to make some provision for their safety. For example, the piratical Barbary corsairs ravaged the south-west coast until the late eighteenth century, destroying hamlets and taking their inhabitants to be sold as slaves in North Africa. It was, nevertheless, fear of invasion from continental powers (from the late seventeenth to the mid-nineteenth century, France, and in the twentieth century, Germany), that occasioned the major defensive fortifications built in the modern period. There was always the possibility that the sea, the Royal Navy and more recently the RAF might not be able to prevent an invasion force getting through and attempting a landing. In response to such an eventuality our coasts are dotted with the lookouts and defensive installations which successive generations built to meet threats that, in the end, never came.
In 1794 the English, with great difficulty, took a small fort at Cape Mortella on Corsica. It was no great fortress but rather a small defensive emplacement capable of giving some protection to soldiers while they fired upon invaders but the British government was impressed and in 1804, when Napoleon threatened to invade, a building programme of similar forts on the south coast was begun. They were designed to mount one gun and be garrisoned by an officer and 24 men and 103 of them were erected in East Anglia, Kent and Sussex. Amongst surviving examples are those at Folkestone, Sandgate and Dymchurch.
Worsening relations between Britain and France in the mid-nineteenth century led to extensive fortifications being ordered by Lord Palmerston's government. "Palmerston forts" were built to defend the Thames and the premier naval ports. Some of these fortifications were actually erected in shallow water and Grain Tower Battery in the Medway is perhaps the most spectacular.
Britain was clearly most vulnerable to an invasion across the channel and it is the south coast which has many of the most considerable fortifications, but the danger that Ireland might fall to an invader made it necessary to install defences on the west coast too. Meanwhile, the whole of the east coast also presented tempting places for invading forces. Improvements to the speed and armaments of warships increased the need for fortifications along the entire coastline. From the late nineteenth century Scottish ports and harbours became significant naval bases which needed static gun placements, a need which increased as Germany rather than France emerged as Britain's main opponent from the early twentieth century. In 1879 three forts were built on Inchkeith Island in the Firth of Forth together with a gun battery on the Fife shore; these defences were to be further improved in both the First and Second World Wars. Scapa Flow, a great natural harbour in the Orkney Islands, became a major naval base at the beginning of the First World War, providing a natural harbour seemingly difficult for submarines to penetrate. However, the sinking of the Royal Oak in 1939 by a German U-boat proved that this wasn't the case and led to a massive programme to block the paths for submarines by booms, causeways, blockships and submarine nets.
Both World Wars saw a massive investment in coastal fortifications. Changes in military technology, particularly the coming of air warfare increased the problems that had faced the defenders of Britain's shores since Roman times but did not fundamentally change them. Early warnings of attack, fortification to resist attack and communications to summon reinforcements remained the essence. It is, nevertheless, astonishing that only a few decades saw, successively, the sound mirrors of Dungeness in 1929, the first radar stations of 1938 and, after the Second World War, RAF Fylingdales, to give early warning of a nuclear attack.
That Britain was not invaded in 1940-41 was due to the Royal Navy and the RAF which maintained command of the sea and enough command of the air so that any attempt at invasion would have been a desperate venture. The coastline of Britain bears witness, however, to the great effort that went into coastal defences: all those thousands of pill boxes (the Martello Towers of the twentieth century), tank traps, trenches and gun emplacements. The past provided more than a back-drop for World War II defences often incorporated or utilised those of previous centuries:
"One pill box suitably camouflaged was built into the Roman walls of Pevensey Castle. The LDV [Home Guard] patrolled the walls of Southsea Castle, built by Henry VIII to keep out the French. There were troops stationed at Landguard Fort, which in 1667 had beaten off the Dutch." - Norman Longmate, Island Fortress
Blyth in North East England demonstrates both continuity and change in preparations for coastal defence. During the French Revolutionary and Napoleonic Wars, thirteen regiments of artillery were stationed along the Northumberland coast. The small port of Blyth bears the imprint of successive gun installations: a battery was established there to meet the threat of invasion from French Revolutionary, and then Napoleonic, forces. It was abandoned after Waterloo but when, in the late 1850s, France appeared a threat once more it was decided to re-establish the old battery and build a new one. In 1916 new defences consisting of search lights and a gun battery were erected. Demolished after the First World War, the battery was recommissioned and extended in 1939 and 1940. Decommissioned in 1949, the gun emplacements survive, much as they were when abandoned. Blyth is far from an extraordinary example for all around Britain the defences of the island over the centuries sit on top of each other.
Clements, Bill, Towers of Strength: the Story of Martello Towers (1999)
Hogg, Ian V. Coast Defences of England and Wales, 1856-1956 (1974)
Longmate, Norman, Defending the Island: from Caesar to the Armada (1989) and Island Fortress (1991)