Definitions of what constitutes traitorous behaviour have changed over time. Before its final complete abolition in the past few years, the death penalty was legally retained (although never used) for a handful of offences deemed too serious to be forbidden by the threat of mere imprisonment. These included treason itself, as well as setting fires in the Naval dockyards (behaviour that was likely to harm the nation’s ability to defend itself, and therefore classed as traitorous).

The focal point for centuries for all treatment of miscreants found to be plotting against the State was the Traitors’ Gate, the Thames-side water entrance to the Tower of London. The gate didn’t always have the sinister function it came to acquire, having been designed initially by Edward I’s architect, Master James of St George, in the 1270s as a convenient riverside entrance to the Tower for when the King chose to arrive by boat. In these days, and until the 16th century, the Tower provided accommodation for members of the royal family.

Gradually, over time, the Gate assumed its infamous reputation as it became the favoured route for committing important prisoners to the Tower, especially those charged with treason. As the accused were conveyed by barge along the river, they would pass under London Bridge, where the severed heads of executed traitors were mounted on poles as a sombre warning to others – a practice that wasn’t finally abandoned until the late 17th century.

Find out more about the Tower of London here.

Anne Boleyn
In 1536, one of the  most notorious episodes in the Tower’s history took place, when Anne Boleyn was transported there through the Traitors’ Gate. Anne had fallen from favour as the second wife of Henry VIII, virtually as soon as the King had laid eyes on Jane Seymour. She was arrested at Greenwich, falsely charged with adultery, incest and treason, and taken to the Tower along the exact route she had taken three years earlier in preparation for her coronation. On arrival, she was informed by the Constable of the Tower, William Kingston, that she would be quartered in the same rooms in which she had prepared herself to be crowned Henry’s Queen. The hearing was short and brutal, and Anne was beheaded in the Tower grounds two-and-a-half weeks later.

Princess Elizabeth
Eighteen years later, Anne’s daughter, Princess Elizabeth, was transported to the Tower in the same way on the orders of her half-sister, Queen Mary. She stood accused of plotting to usurp the Queen. Elizabeth spent a terror-stricken two months incarcerated in the Tower before her exoneration and release. She went on to become one of England’s most celebrated monarchs, Queen Elizabeth I.

Robert Devereux
In 1601, Elizabeth presided over the execution for treason of her former favourite Robert Devereux, Earl of Essex. Fearing an uprising in support of the Earl, the Queen readily acceded to his request to be executed privately rather than in public.

Roger Casement
The Irish rebel leader Roger Casement was imprisoned in the Tower during proceedings against him in 1916. A British diplomat in the days when Ireland was still a constituent part of the United Kingdom, he stood accused of procuring arms from Germany – during the first world war – to help fight the cause of Irish independence.

The Treason Act did not permit charges to be brought against those who had committed treacherous acts abroad, and so a creative reinterpretation of the original medieval documents was undertaken in order to try him. Convicted under these highly dubious legal circumstances, Casement was hanged at Pentonville prison despite a high-profile campaign for clemency. In the 1960s, his remains were repatriated to Ireland, and reinterred with full national honours in Glasnevin cemetery in Dublin.

Norman Baillie-Stewart
One of the last British citizens to be detained in the Tower of London for treason was Norman Baillie-Stewart. He was found guilty in 1933 of selling military secrets to Germany, and sentenced to five years’ imprisonment. Released in 1937, he immediately fled to Austria, but was denied naturalisation.

Baillie-Stewart became a German citizen in 1940, and was involved – along with William Joyce (see below) – in the preparation and delivery of wartime propaganda broadcasts for the Nazi government. At the end of the war, he was arrested and again accused of treason, on the grounds that he had still technically been a British citizen at the outbreak of the war. Since relevant documents had been mislaid, Baillie-Stewart was charged with the lesser offence of committing acts likely to assist the enemy, convicted and sentenced to five years in prison. On his release, he lived out his remaining days in Dublin, dying there in 1966 at the age of 57.

William Joyce
At the same time as Baillie-Stewart was found guilty, William Joyce was sentenced to hang. Joyce, better known as Lord Haw-Haw, was an Army officer who had joined Oswald Mosley’s British Union of Fascists in 1932, eventually becoming the party’s deputy leader. His reputation as a street brawler eventually put Mosley off, and he was sacked, leaving to set up his own breakaway party in the process.

Just before the outbreak of war, Joyce received a tip-off from within MI5 that he was about to be detained, and fled to Nazi Germany with hs wife. He became a naturalised German citizen in 1940. Throughout the war, Joyce made propaganda broadcasts on behalf of Hitler, urging the British people to give up the struggle. Although it was an offence to tune in to these broadcasts, many people in Britain did so, finding an eerie fascination in the contemptuous sneer in which they were delivered. Each one began with the sombre trademark greeting, “Germany calling, Germany calling,” and ended with a loyal “Heil Hitler!” The Führer decorated him with a national award for his services.

Joyce’s final broadcast was made days before the German surrender. He sounded quite broken and unmistakably drunk, and reproached the British people for their part in the imminent destruction of the Nazi regime. Although Joyce was American by birth, he was tried on the grounds that he had held a British passport, for all that it was fraudulently obtained. It was declared that its possession had enabled Joyce to take advantage of British diplomatic protection. Convicted of aiding the King’s enemies, he was hanged at Wandsworth prison in January 1946, the penultimate person ever to be executed in the UK for a crime other than murder.

The Cambridge spies
The most famous traitors of the 20th century were the Cambridge spies. Members of the intelligence services who had been recruited by agents of the former Soviet Union before and during the second world war, they were exposed piecemeal only after they had done considerable damage to the UK’s own espionage network. They were Guy Burgess, Donald Maclean, John Cairncross, Kim Philby and Sir Anthony Blunt.

Of the five, Burgess, Maclean and Philby defected to the Soviet Union, the first two in 1951 and Philby in 1963, only after leaving the intelligence service and working as a journalist for some years in the Middle East. Cairncross and Blunt were granted immunity from prosecution in return for confessing and naming names. Blunt’s 1964 confession was publicly confirmed in 1979, while Cairncross’s – originally made in 1951 – was acknowledged only in 1990.

Cairncross went on to a career with the UN in Rome. Blunt became Surveyor of the Queen’s Pictures and a director of the Courtauld Institute of Art and, at the time of his unmasking, was still a Fellow of Trinity College, Cambridge, a post from which he had to resign.

Two plays by Alan Bennett, An Englishman Abroad (1983) and A Question Of Attribution (1988), document the cases of, respectively, Burgess and Blunt.

Melita Norwood
In 1999, one of the more striking Soviet agents was unmasked. Melita Norwood, then 87, was an apparently unassuming elderly lady living in Bexleyheath, who had been a Soviet agent from the 1930s until the late 1970s. While employed at the British Non-Ferrous Metals Research Association, which was involved in the early stages of the development of British nuclear weapons, she systematically began passing copies of documents to the Russians. She achieved this by the astonishingly low-tech means of slipping the relevant papers off her superiors’ desks while they weren’t looking, photographing them and handing them to her contacts at secret trysts in the London suburbs.

In 1979, Norwood and her husband were accorded the red-carpet treatment on a visit to Moscow. She had been awarded the Order of the Red Banner in 1970 for her services to the Soviet state, and granted a KGB pension of £20 a month. When her past was revealed in an article in The Times, she told reporters that she had been motivated by socialist conviction (“I loved Lenin!” she declared), as well as the desire to see the USSR given an equal chance in the nuclear stand-off between the superpowers that dominated the Cold War.

The government decided against prosecuting her, and she died peacefully and unrepentantly in a nursing home in 2005 at the age of 93. She had been the longest-serving female spy the Soviet Union ever recruited in the UK. “I thought I’d got away with it,” she said, when journalists first arrived on her doorstep. Effectively, she had.