The caves of the Mendip Hills were settled during the Palaeolithic period, and contain extensive archaeological sites such as those at Cheddar Gorge. Bones from Gough's Cave have been dated to 12,000 BC, and a complete skeleton, known as Cheddar man, dates from 7150 BC. Examples of cave art have been found in caves such as Aveline's Hole. Some caves continued to be occupied until modern times, including Wookey Hole.

The Somerset Levels—specifically the dry points such as Glastonbury and Brent Knoll— also have a long history of settlement, and are known to have been settled by Mesolithic hunters. Travel in the area was helped by the construction of one of the world's oldest known engineered roadways, the Sweet Track, which dates from 3807 BC or 3806 BC.

The exact age of the henge monument at Stanton Drew stone circles is unknown, but it is believed to be Neolithic. There are numerous Iron Age hill forts, some of which, like Cadbury Castle and Ham Hill, were later reoccupied in the Early Middle Ages.

On the authority of the future emperor Vespasian, as part of the ongoing expansion of the Roman presence in Britain, the Second Legion Augusta invaded Somerset from the south-east in AD 47. The county remained part of the Roman Empire until around AD 409, when the Roman occupation of Britain came to an end. A variety of Roman remains have been found, including Pagans Hill Roman Temple in Chew Stoke, Low Ham Roman Villa and the Roman Baths that gave their name to the city of Bath.

After the Romans left, Britain was invaded by Anglo-Saxon peoples. By A.D. 600 they had established control over much of what is now England, but Somerset was still in native British hands. The British held back Saxon advance into the south-west for some time longer, but by the early eighth century King Ine of Wessex had pushed the boundaries of the West Saxon kingdom far enough west to include Somerset. The Saxon royal palace in Cheddar was used several times in the 10th century to host the Witenagemot. After the Norman Conquest, the county was divided into 700 fiefs, and large areas were owned by the crown, with fortifications such as Dunster Castle used for control and defence. Somerset contains HMP Shepton Mallet, England's oldest prison still in use, which opened in 1610. In the English Civil War Somerset was largely Parliamentarian, with key engagements being the Siege of Taunton and the Battle of Langport. In 1685 the Monmouth Rebellion was played out in Somerset and neighbouring Dorset. The rebels landed at Lyme Regis and travelled north, hoping to capture Bristol and Bath, but they were defeated in the Battle of Sedgemoor at Westonzoyland, the last pitched battle fought in England. Arthur Wellesley took his title, Duke of Wellington from the town of Wellington; he is commemorated on a nearby hill by a large, spotlit obelisk, known as the Wellington Monument.

The Industrial Revolution in the Midlands and Northern England spelled the end for most of Somerset's cottage industries. Farming continued to flourish, however, and the Bath and West of England Society for the Encouragement of Agriculture, Arts, Manufactures and Commerce was founded in 1777 to improve farming methods. Despite this, 20 years later John Billingsley conducted a survey of the county's agriculture in 1795 and found that agricultural methods could still be improved. Coal mining was an important industry in north Somerset during the 18th and 19th centuries, and by 1800 it was prominent in Radstock. The Somerset coalfield reached its peak production by the 1920s, but all the pits have now been closed, the last in 1973. Most of the surface buildings have been removed, and apart from a winding wheel outside Radstock Museum, little evidence of their former existence remains. Further west, the Brendon Hills were mined for iron ore in the late 19th century; this was taken by rail to Watchet Harbour for shipment to the furnaces at Ebbw Vale.

Many Somerset soldiers died during the First World War, with the Somerset Light Infantry suffering nearly 5,000 casualties. War memorials were put up in most of the county's towns and villages; only nine, described as the Thankful Villages, had none of their residents killed. During the Second World War the county was a base for troops preparing for the D-Day landings. Some of the hospitals which were built for the casualties of the war remain in use. The Taunton Stop Line was set up to repel a potential German invasion. The remains of its pill boxes can still be seen along the coast, and south through Ilminster and Chard.

A number of decoy towns were constructed in Somerset in World War II to protect Bristol and other towns, at night. They were designed to mimic the geometry of "blacked out" streets, railway lines, and Bristol Temple Meads railway station, to encourage bombers away from these targets. One, on the radio beam flight path to Bristol, was constructed on Beacon Batch. It was laid out by Shepperton Film Studios, based on aerial photographs of the city's railway marshalling yards. The decoys were fitted with dim red lights, simulating activities like the stoking of steam locomotives. Burning bales of straw soaked in creosote were used to simulate the effects of incendiary bombs dropped by the first wave of Pathfinder night bombers; meanwhile, incendiary bombs dropped on the correct location were quickly smothered, wherever possible. Drums of oil were also ignited to simulate the effect of a blazing city or town, with the aim of fooling subsequent waves of bombers into dropping their bombs on the wrong location. The Chew Magna decoy town was hit by half-a-dozen bombs on 2 December 1940, and over a thousand incendiaries on 3 January 1941. The following night the Uphill decoy town, protecting Weston-super-Mare's airfield, was bombed; a herd of dairy cows was hit, killing some and severely injuring others.

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