Calais history may have been dating to ancient times but proper evidence of human settlements is from the Roman era when it was known as Caletum. In medieval times, it was a part of a Dutch-speaking area of France and was then known as Kales. It is the last exit point of France and the first northern entry point from the viewpoint of England, it bloomed into an important port, from the time it was a fishing village in the 10th century. By the 13th century in 1224, it was fortified by the Count of Boulogne.
Due to its strategic location and importance in ferry transport with Dover, the Port of Calais was always hounded by many, to grab it for furthering their prospects of trade. Edward assumed himself to be the heir to the Kingdom of France, after his uncle Charles IV's death in 1328. But the French thought otherwise and chose the line of descent from his great grandfather. So the House of Valois was chosen as the heir to the French throne, leading to a military brush between the English and Welsh. Finally, it was invaded by King Edward III of England in 1347 and seized after an eleven month-long Battle of Crecy where the locals lost their town.
As an act of vengeance, King Edward demanded payback from the local citizens for not surrendering to him immediately during the battle. He ordered that a mass eradication of population in the form of massacre would take place and they would be spared if six important citizens of the town came forward and surrendered themselves to die and that too barefooted and bareheaded with ropes around their necks. But upon Queen Philippa of Hainault request, the six of them were spared. This historical episode is reconstructed in the form of statues of The Burghers of Calais (Les Bourgeois de Calais) erected in 1888 and created by famous sculptor Auguste Rodin.
Even if the six citizens were spared, later on, King Edward expelled most of the French citizens and the town was settled by mostly English people for the only reason that it would serve as a connecting port between France and England.
With the Treaty of Brétigny in 1360, Guînes, Marck and Calais are jointly known as "Pale of Calais" was unofficially allotted to the English and only partly put into practice. In 1363 it became an important port and in 1372 became a constituency of the Parliament of England. But it did keep its bonding with France as it still was a part of the diocese of Therouanne.
With it becoming attached to the Parliament of England, Calais came to be known as the "brightest jewel" in the English Crown as the port revenues amounted to 1/3rd of England's revenue. Primarily, tin, lead, cloth and wool were the chief trade items. In fact, nearly half of the population of Calais was connected to the wool trade.
For many years Calais remained an important part of the English Parliament with its representative being a part of government rule. But it was turning out to be an expensive proposition to maintain Calais as it didn't have any natural defences and so had to depend on man-made fortifications.
The border of Calais and Franco-Burgundy were the same and so there was a constant threat from French and Duchy of Burgundy forces. Both the French and Duchy of Burgundy hated each other so much that they preferred Calais with England rather than be it a part of each other's constituency. But finally, the French overtook the Burgundy and incorporated it into France.
When Francis, Duke of Guise took over the throne of France, he took benefit of the disadvantage that Calais had regarding natural defences. On January 1, 1958, the English lost their rule over Calais. The French attacked Calais at the city's weak point of Fort Nieulay and its gates which when opened could have sent the French back. But the gates remain closed, thus surprising the English. Calais (then known as Calaisis) was renamed as Pays Reconquis and the Dutch citizens were forced to speak French. The city came under the rule of Spanish for a short period of time from 1596-1598 when it was later returned to the French under the Treaty of Vervins.
Fast forward, during World War II, Calais was devastated by the German and French troops. In the War of Siege of Calais in 1940 which preceded Operation Dynamo, the 10th Panzer Division German forces held out for many days at the city against the allied forces of French and English. This led to the bombing and precise attacks by both sides of the forces, leading to the destruction of the city.
During the German occupation, it was heavily fortified and used as a base for the launch of flying bombs, and placement for railway guns. But despite all the preparations, the Allied forces planned their attack well and bombed Calais heavily and with the help of Canadian forces, Calais was liberated in 1944.