While publicity boasted that Cromwell was made "after ten years of research", many liberties were taken with what actually happened.
- The controversial enclosures
appear early on as one of the reasons for Oliver Cromwell's discontent. In reality, Charles I was notably anti-enclosure. - Cromwell and Henry Ireton are said to be among the five Members of Parliament King Charles I tried to arrest when he entered the House of Commons. In reality, neither was among the MPs the King wanted to be detained.
- One fact the movie never touches on is that Ireton was Cromwell's son-in-law, having married his oldest daughter Bridget.
- Though the movie shows both the Earl of Essex and the Earl of Manchester in the House of Commons with Cromwell, they were actually members of the House of Lords.
- Roundheads are depicted as being much more plainly dressed than the Cavaliers. To be fair, there was some truth to this; however, the fact that sashes and other insignias of allegiance are known to have been needed strongly indicates that this was hardly a universal rule, or at least not as stark a contrast as the movie presents it.
- Cromwell tells Charles that he believes England should have a democratic government. It's generally agreed that he made no such suggestion to the king; furthermore, the extent to which Cromwell believed in democracy (even as it was understood during his lifetime) is debated.
- The movie shows Cromwell giving the famous soldier's prayer: "O Lord, Thou knowest how busy I must be this day. If I forget Thee, do not Thou forget me". This was actually a prayer used by the Royalists.
- Unlike in the movie, the infantry of the New Model Army didn't wear black and gold coats. They wore red ones.
- While the movie shows Cromwell as a colonel at the Battle of Edgehill, he was only a captain at the time. Moreover, he and his men arrived too late in the day to participate in the battle.
- Prince Rupert's white hunting poodle Boy appears shortly before the Battle of Naseby. In reality, Boy was killed at Marston Moor a year earlier.
- For some reason, the Battle of Marston Moor — the largest individual engagement of the Civil War — never even gets mentioned.
- A scene where Cromwell visits his son's grave indicates that the Battle of Naseby happened in 1644. It actually took place in 1645.
- Unlike in the movie, Cromwell was not the commander-in-chief of the New Model Army during the Civil War. That was Sir Thomas Fairfax; Cromwell was his second-in-command.
- While Cromwell personally arrests the King at Oxford in the name of Parliament, in reality, Charles surrendered to the Scottish Presbyterians and was only handed to the Parliamentarians after months of negotiations.
- Though John Pym is reported dead in 1646, the real John Pym died three years earlier in 1643.
- Edward Hyde is called "Sir" and addressed as "my lord" in scenes taking place in 1641. Hyde was not knighted until 1643, and not ennobled until 1661.
- In the movie, Charles I is put on trial merely for planning a second civil war, with his plans being discovered before they can be carried out. But in reality, this second civil war was fought, and it was only after Charles was defeated in it that he was put on trial.
- Cromwell and Fairfax are shown bringing troops into the House of Commons and declaring that Cromwell now has a majority. While an event similar to this known as Pride's Purge really did happen, Cromwell wasn't there when it occurred and it's not clear whether he had any advance knowledge of it.
- Sir Thomas Fairfax is shown as one of the judges at Charles's trial. While Fairfax did turn up for the first day of trial, he absented himself afterwards.
- At the king's trial, Hyde gives damning testimony. But Hyde was not present for the trial in real life by virtue of being out of the country at the time.
- Cromwell is the first to sign Charles's death warrant after Fairfax refuses to do so. In actuality, Cromwell was the third to sign.
- Though Ireton is shown as one of the delegations of MPs who offer to make Cromwell King of England, the real Ireton had been dead for six years by the time the offer was made.
- When Cromwell is offered the throne of England, he dismisses the idea almost immediately. Though the real Cromwell was certainly uncomfortable with the idea of becoming king, he took the offer very seriously and only turned it down after weeks of negotiations.
- Near the end of the movie, Cromwell tells the Rump Parliament they've had six years to make a new government after Charles was executed. In reality, they had four. Moreover, this scene takes place after Cromwell is offered the crown, which didn't happen until eight years after Charles's execution.
- The Earl of Essex is present in the last scene when Cromwell dissolves the Rump Parliament, which takes place in 1653. However, the Earl of Essex died in 1646.
