Bermuda

Martello Tower, located at the Ferry Point in St. George's Bermuda, was built by the British Army at the time of Major Thomas Blanshard.  The construction of the fort started in 1823 and completed in 1828. It was built to protect Ferry Reach, a channel that runs from St George Harbor to Castle Harbor. 
 
The fort looks like a cylindrical egg-shaped tower. It's made of hard Bermuda stones. The walls are 9-11 ft. thick & resistant to cannon fire. The only access to the tower is through a narrow drawbridge over a moat to the second level. The British had built many such Towers in other countries during the 19th century. But this was the only one built in Bermuda. 

Canada

Carleton Martello Tower in Saint John, New Brunswick, is one of the nine surviving Martello Towers in Canada. The tower dates from the War of 1812 and played a significant role in conflicts until the Second World War. The site now features a restored powder magazine, a restored barracks room, and exhibits in the tower and in the visitor centre. The tower's roof offers a view of the city of Saint John and its harbour. Carleton Martello Tower is the oldest building in the city and has been designated as a National Historic Site of Canada since 1930. It was opened to the public in 1963.

United States

The United States government built several Martello towers at locations along the eastern seaboard. Two are at Key West, Florida; others were built at the harbours of Portsmouth, New Hampshire, Charleston, South Carolina and New York City. Two more Martello towers stood at Tybee Island, Georgia and Bayou Dupre, Louisiana.

Although the Americans copied the design from the towers the British erected in Canada, the American Martello towers differed in some significant respects from the British. The Martello tower built at Tybee Island, Georgia was constructed around 1815 utilizing wood and tabby, a common local building material at the time, instead of the brick or stone that the British towers used. Also unlike the British towers, the Tybee tower featured gun loops on the garrison floor that enabled muskets to be fired through the walls.[6] It was never tested in battle and by the time of the American Civil War was in a state of disrepair. Its unfamiliar design confused local writers, who often said that the Spanish had built the tower when Georgia was Spain's colony.

The Key West towers, though the locals refer to them as Martellos, were square instead of round and had thin walls with long gun loops. In addition, a curtain wall of heavy guns encircled the Key West towers making them, effectively, keeps instead of standalone towers.

Today only two Martello Towers still exist in the United States, both are located in Key West although the towers differ in design from the traditional towers. Both Martello Tower East and West feature somewhat of a square design rather than the traditional round plans. One has been converted into a botanical garden while the other is now a museum featuring a Civil War exhibit dealing with the role of Key West and the surrounding area during the war.