Factual error: In the scene where the king attempts to seize the five members from Parliament, Cromwell makes a dramatic refusal to leave and proposes various "Laws" to prevent his arrest. Cromwell was not one of the five members whom the King tried to arrest and no law can come into force until it had been signed by the reigning Monarch anyway.
Suggested correction: It's more of a threat than a declaration.
Factual error: Sir Thomas Fairfax was in overall command of the New Model Army at Naseby, not Cromwell. He was in charge of the cavalry on the right wing (the Ironsides).
Factual error: In the scene of the Battle of Edgehill (the first battle in the movie) Cromwell utters the famous "Oh Lord, Thou knowest this day..." prayer. This was, in fact, said by Sir Jacob Astley one of the Royalist Generals on the other side. It is also unlikely that Cromwell was even at the battle.
Continuity mistake: At the battle of Edgehill when Crowell's cavalry and Prince Rupert's cavalry are in the middle of a melee and you can see a wound on Crowell's left arm, the same wound he receives at Naseby. It is clear that the scene was meant to be used for the battle of Naseby but was cut and used at Edgehill.
Factual error: Halfway through the film, Cromwell decides to create the New Model Army, and he kits them all out in dark brown uniforms. In fact, Cromwell's New Model Army wore red coats, (cheapest available colour), and this was the reason the British army wore red until the late 19th Century. In the movie, King Charles's troops all wear red; in fact each different regiment wore their own coat colours, blue being a popular choice.
Factual error: In the last battle, Naseby, the Parliamentarians are shown to be outnumbered by the Royalists. In fact it was the other way round (13,000 to 9,000).
Factual error: The movie shows that Charles I remained in Oxford until Oliver Cromwell himself arrested him. In reality, Charles fled to Scotland but the Covenanters sold him back to Parliament.
Factual error: The movie climaxes with Cromwell driving out the rump and toppling the House of Commons' ceremonial mace of authority. In reality, he said "What am I to do with this?" and had the mace taken away.
Factual error: In real life, Cromwell was not the first to sign Charles I's death warrant but the third. The first signatory was John Bradshaw, the judge who sentenced Charles to death.
Factual error: Charles I never exiled Prince Rupert face to face. He did it by letter.
Factual error: Many things are wrong in the scene where Charles I storms the House of Commons. He and Oliver Cromwell never met face to face. Cromwell was not one of the men who was to be arrested. Cromwell's threats were said by John Hamden. Hamden was also to be arrested except for the fact that he had died ten years before.
Factual error: The Battle Of Edgehill (on 23 October 1642) was the first full-scale, pitched battle of the Civil War. Although not named as such, this is shown as a total rout for the Parliamentarians/Roundheads, no match for the dashing Royalists/Cavaliers. Cromwell tries to rally his men, but, realising the situation is hopeless, he, too, leaves the field. In historical fact, the Royalist army, advancing on London, met the Parliamentary army at Edgehill. The battle was an inconclusive draw. Both armies suffered equal casualties. Neither side gained or was driven from the field. After Edgehill the Parliamentary army retreated to London. That may have appeared defeatist, but was actually a wise thing to do. The Royalists could have occupied an undefended London, but lacked supplies and material to besiege London if it was defended. Thus, the Royalist army advanced to the outskirts of London but had to withdraw after a few days, after which the Parliamentarians held London for the rest of the war. Cromwell did not fight at Edgehill. On 23 October he was leading a troop to join the main Parliamentary army and did not link up until several days later.
Factual error: Peers and commoners (like Cromwell) are seen sitting in the same chamber in Parliament. In reality, of course, the Houses of Commons and Lords are separate chambers.
Factual error: The movie says that John Pym, the leading parliamentarian reformer, died after the war and his followers go to Oliver Cromwell to help oust the corrupt rump. In reality Pym died during the war.
Factual error: The movie shows that Oliver Cromwell went home after King Charles I's execution until he was called back to drive out the rump. In reality, he was in his infamous Irish pacification campaign to put down the rebellion that had been going on there since before the civil war.
Continuity mistake: During Prince Rupert's first appearance, he greets his uncle King Charles I by doffing his hat and putting it back on. In the close-up shot he is holding his hat in place but in the wide shot his hand is on his side.
Factual error: At the end of the film the Earl of Manchester is shown setting up corrupt, self-seeking government that will be much worse than the royal regime it has replaced. An enraged Cromwell calls soldiers to drive the Earl of Manchester and his sycophantic supporters from the Houses of Parliament. This scene completely re-writes history. The Earl Of Manchester, was what would now be termed a 'moderate' supporter of the Parliamentary cause, who hoped that Charles I would remain king, but with reduced powers. He did not want to replace the monarchy, or to take over as ruler of the country (in the way that Cromwell eventually would). As Cromwell's power grew the Earl of Manchester's role in affairs shrank. At Charles I's execution he withdrew from government until Cromwell's death, when he assisted the Restoration of Charles II, becoming one of Charles II's most loyal supporters.
Continuity mistake: When Oliver Cromwell purges an altar of "idolatrous" candlesticks, he begins by knocking down the second from the left. In the close-up, the candlestick is standing again and he has to knock it down a second time. This happens twice in the same scene.