French were captured, and some were sent to France to be used as galley slaves. Denonville's troops and native allies went on to attack the Seneca. In retaliation for these incidents the Iroquois laid siege to Fort Frontenac and blockaded Lake Ontario. The fort and the settlement at Cataraqui were besieged for two months in 1688. Although the fort was not destroyed, the settlement was devastated and many inhabitants died, mostly from scurvy. The French abandoned and destroyed the fort in 1689, claiming that its remoteness prevented proper defense and that it could not be adequately supplied. The French again took possession of the fort in 1695 and it was rebuilt and strengthened to serve primarily as a military base of operations. From Fort Frontenac in 1696 the French organized an attack on the Iroquois who inhabited areas south of Lake Ontario. Increased tension between the British and the French in the 1740s led to the French upgrading the fort's defensive capabilities by adding new guns, building new barracks and increasing the size of the garrison. However, when the Marquis de Montcalm arrived at the fort in 1756 to launch an attack on the British at Oswego, he was not impressed with its construction. One of his engineers noted that: The fort's strategic significance gradually decreased. Other forts such as Fort Niagara, Fort Detroit, and Fort Michilimackinac became more important. By the 1750s Fort Frontenac essentially served only as a supply storage depot and harbour for French naval vessels, and its garrison had dwindled. Battle of Fort Frontenac During the Seven Years' War between Britain and France, who were vying for control of the North American continent, the British considered Fort Frontenac to be a strategic threat since it was in a position to command transportation and communications to other French fortifications and outposts along the St. Lawrence – Great Lakes water route and in the Ohio Valley. Although not as important as it once was, the fort was still a base from which the western outposts were supplied. The British reasoned that if they were to disable the fort, supplies would be cut off and the outposts would no longer be able to defend themselves. The Indian trade in the upper country (the Pays d'en Haut) would also be disrupted. Fort Frontenac was also regarded as a threat to Fort Oswego, which was built by the British across the