Windsor was where Oliver Cromwell and his colleagues finally decided once and for all that King Charles I whould have to go. According to Christopher Hibbert’s book “Cavaliers and Roundheads” it was at a prayer meeting of Army officers in Windsor in late 1648 that they agreed (many with tears in their eyes) to bring Charles Stuart ‘to account for the blood he had shed’. Until that point most of the parliamentarians, including Cromwell himself, had expected to come to some sort of accomodation with the King. Shortly thereafter, Cromwell’s own son in law who was at Windsor at the time, produced a document called ‘Remonstrance of the Army’ which explicitly called for abolishion of the Monarchy and for Charles himself to be brought to trial.
The population of Windsor would probably have been very much in agreement with the Remonstrance, as there was a lot of anti-monarchist feeling in the town due to the strict imposition of forest laws, regular prosecutions for poaching and various acts of enclosure carried out by the Crown.
Ultimately, Charles was tried and executed in London and his body brought back to Windsor where it was buried in secret in the crypt of St. George’s Chapel.