Louisburg (lōˈĭsbərgˌ) [key], town (1991 pop. 1,261), E Cape Breton Island, N.S., Canada. The town, an ice-free port, is near the site of the great fortress of Louisbourg, built (1720–40) by France as its Gibraltar in America. Plans were drawn by the great French engineer Vauban, but the work was poorly done, and the garrison was inadequately supplied and at odds with the civilian population. French privateers, using the harbor as a base, preyed on New England fishermen working the Grand Banks, until 1745, when a small force of New Englanders under William Pepperrell, supported by a fleet of merchantmen commanded by Sir Peter Warren, attacked Louisbourg and forced its surrender. Three years later it was returned to France by the Treaty of Aix-la-Chapelle, in exchange for Madras (now Chennai), India, but it fell (1758) to a British land and sea attack led by Gen. Jeffery Amherst and Admiral Boscawen, which reduced it to ruins. The site is a national historic park, and reconstruction of a portion of the fortified town is completed.