Calais was an important little spot to the English, since it served as a toehold on the mainland and a trading center for English wool that allowed English merchants to bypass the markets in the low countries (present day Netherlands). When the French tried to meddle in trade in Calais, the English retaliated and the Battle of Crecy was fought in 1346. After the English triumphed in the battle, King Edward III beseiged the town of Calais for eleven months, finally capturing it. He threatened to have the town’s entire population put to death, as punishment for holding out so long (which would surely have badly affected the local economy, wouldn’t it?) but was persuaded to let the townspeople off the hook. He did, however, drive most of the French from the town and settled it with people from England. By 1360, Calais and the nearby areas of Guines and Marck were granted to English rule in perpetuity by the Treaty of Bretigny.
Over time, Calais became known as the “brightest jewel in the English crown.” Revenues from its trade in tin, lead, cloth and wool amounted to as much of a third of the English government’s revenue.
Despite its importance, Calais was rather neglected after the mid-16th century. Mary I was too busy trying to eradicate Protestantism in England to notice that Calais’s fortifications were in poor repair and the garrison was weak. Francis, Duke of Guise noticed, however, and attacked on January 1, 1558. They took the city a few days later, and the loss was considered a huge black mark on Mary’s already heavily blighted reign.