I saw three of these.
They are Sentry Boxes (also known as Pill Boxes). They were built during World War 2 to houuse troops in case of a German Invasion. The heavy construction was to make them bomb resistant, and the small apertures were to allow occupants to fire on the enemy whilst affording a degree of protection. You may have also noticed some large square concrete blocks positioned at some of the bridges where famr track cross the canal on this stretch. They were put there to hider the progress of enemy tanks in the event of an invasion making landfall on the North West Coastline.
these were pill boxes and housed anti aircraft guns during the war to protect the boats from german attacks.