Martello towers guard the mouth of the Rideau. There are actually three in the picture, the third hidden by the trees to the left. It stands out in the mist better here. Built in the 1840s to protect the then high-tech canal technology of the Rideau accessed between the nearest two towers, the theory was similar to Halifax Harbour’s defences – a killing zone of miles deterring any thought of attack. One gun in the red roofed tower to the left had a range of 7 miles. Greater detail and a harbour map can be found here.