This is the Blockhouse at the head of the locks. That's Colonel by the Lake in the background which leads to the upstream locks leading to Ottawa.

If you look at the URL on the Gallery page you will see a picture that shows the Blockhouse having a clear view down the locks and downstream on the Cataraqui River.

I suspect the cinder blocks on the bottom on the Blockhouse are an upgrade over likely the original stones. You can see shooting slots (correct name?) on the right.