There is a Martello located at Ferry Reach in St George's Parish. It was completed in the 1820s.
Martello towers (or simply Martellos) are small defensive forts built in several countries of the British Empire during the 19th century, from the time of the Napoleonic Wars onwards.
They stand up to 40 feet (12m) high (with two floors) and typically had a garrison of one officer and 15-25 men. Their round structure and thick walls of solid masonry made them resistant to cannon fire, while their height made them an ideal platform for a single heavy artillery piece, mounted on the flat roof and able to traverse a 360° arc. A few Martello towers were surrounded by a moat for extra defence. They were used throughout the 19th century, but became obsolete with the introduction of powerful rifled artillery. Many have survived to the present day, often preserved as historic monuments.
Now if I could only get in! (last time was over 25 years ago)
Spoke to the Parks Department, looks like I can get in, just need to arrange a date.