The island itself is in the St. Lawrence. The marker, in a parking area on the river side of the road, is at 1186 Thousand Island Parkway, Mallorytown, Ontario. Google Maps may direct you to the other side of the road.
Commemoration
During the War of 1812 the St. Lawrence was the life-line of Upper Canada along which virtually all military and civilian supplies were transported from Montréal to Kingston. Fear that the Americans might attempt to block the passage of materiel prompted the fortification of Bridge Island as a shelter for the supply batteaux and a base for British gunboats. A blockhouse was completed early in 1814 and a circular battery with an 18-pounder constructed. These defence works were maintained by a detachment of the 57th Regiment and artillerymen during 1814, but fell into disrepair soon after the war.
Background
An 18-pounder is a cannon that was considered intermediate calibre at the time. It was most often used on ships of the Royal Navy.
One cannot overestimate the impact of the St. Lawrence River as a means of transportation and communication. The back country roads, where they existed at all, were the only alternative. They were often impassable due to weather. Loss of the river communications would have left the communities of Upper Canada isolated and easily taken by the Americans at their leisure.