Dover is a town on the coast in Kent, in the United Kingdom. It was built by the Romans. There is a castle in Dover, called Dover Castle. It is the largest castle in England. The population is about 39,078. Throughout history, it has been an important port of Britain because it is the closest port to mainland Europe. The sea between Dover and the French port of Calais is called the Strait of Dover. It is the narrowest part of the English Channel at only wide.