A large stone is catapulted through the stain glass cathedral window. All later scenes show the window fully intact.
When Marks is killed he is shown being brought down by an archer's arrow, but when his body is shown later it is a crossbow bolt in his neck.
In 1215, Rochester was already a sizable city. When Rochester castle is seen in the film, there is no sign of the dwellings that would have comprised the city, nor of the cathedral, which is a massive building, situated about one hundred yards from the castle. The cathedral was looted by King John's forces, during the siege.
The film's final voice-over narration claims that the rebellion against King John was victorious within a year of John's death in 1216, when the exact opposite is true. Royalists fighting on behalf of John's young son, Henry III, defeated the rebellious barons and their French allies in 1217, and Henry went on to rule England for 56 years. Louis, the French prince the film implies succeeded John as king, never held the English throne.
The film shows forty pigs being herded into the mine under the castle walls, and then burned to make the timber pit-props collapse, bringing down part of the castle. Live pigs are actually rather hard to set fire to. What actually happened is that forty pigs were slaughtered, and their fat was taken into the mine, along with other inflammable material such as straw.
The Danes are seen painting their faces blue. This was a Pictish/Scots practice. The blue pigment was extracted from the woad plant (as seen in Braveheart). It was not used outside of what is now the British Isles.
The film implies that the siege lasted for several months. In fact it only lasted seven weeks, from 11 October to 30 Nov 1215.
King John orders 40 pigs to make the fire in the tunnel. However, there are only two transports arriving. In each of these transports only about 5-6 pigs would fit.
There are a lot of Hungarian stunts in the film, who do shout things in the heat of fights, thankfully the meaning does go with what is happening, and if you don't speak Hungarian you probably don't even notice it, but for Hungarian ears its really funny when you watch it and suddenly someone shouting in your language when you wouldn't expect it.
Several stuntmen in the movie are Hungarian, as obvious from the things they shout in the heat of fighting / in Hungarian, in the otherwise Hindi movie.
At one point Marshal wears a helmet of a type known as a 'great helm', a form not used until many decades after the film is set.
King John's mercenaries are a band of "pagan Danes" several hundred years after the whole of Denmark was converted to Christianity.
In the final scene where the horses are pulling the wagons through the mud the leather traces are connected to the cart by marine type stainless steel quick release shackles and mountaineering type stainless steel screw karabiners.
The Danes use armour and equipment which is (very loosely) based on that used by their Viking ancestors more than a century-and-a-half earlier. In fact, in the 13th century Danish soldiers would have been equipped much like the warriors of any other European country.
When discussing why King John needs pigs, Beckett refers to the King's chief miner as a sapper, which is a term for a combat engineer who demolishes fortifications. However, the word sapper did not originate until the 17th Century, meaning that it is incorrectly used in this film as it is four centuries too early.
The Danes speak Hungarian.
When Isabel is tending Marshall's neck after the first fight, she says, "There it is," but her lips don't move.
The film shows Rochester Castle standing next to a bridge in a totally empty moorland landscape. In fact the castle was (and is) on the edge of the City of Rochester, which was already a thousand years old at the time of the siege, and right next to it is the great 11th-century cathedral of Rochester. On the other side of the bridge was (and is) the town of Strood, plus a number of smaller settlements.