The Martello Tower -- One of Great Britain's 'anchors of empire', towers of this design were placed all over the British Empire at its height. While essentially useless after the advent of heavy rifled artillery, they were highly resistant against the smoothbore cannons carried by most warships. Even after the advent of wall-busting artillery, the towers could function as outposts, as they were more than sufficient against rifle fire.
Derived from a Corsican fortification of Genoese design, the Torra di Mortella, which had successfully fought off an attack by a Royal Navy ship-of-the-line and a frigate in 1794, the British were impressed enough by the design that they copied it, and began building copies around their vast empire...all they really got wrong was the name - instead of "mortella", it became "martello".
In its final form, the tower is usually around 40ft/12m in height, with two floors over a water tank and basement storeroom. Atop the roof was a fairly heavy cannon - usually firing a 24-pound shot - mounted on a pivoting carriage with a 360° traverse. Towers were rated for a garrison of 24 men and 1 officer.
The towers were comparatively cheap and effective - of the roughly 140 towers built be Britain directly, as well as numerous towers built to the same design by colonies (even the United States got into the act, building a few on its eastern seaboard, and even making onto the heraldic arms of the US Army's 41st Infantry Regiment), only two are known to have fallen to enemy attack: the original Corsican tower (ultimately captured by ground forces after a short siege of the ill-supplied structure) and the Monning Tower, part of the defenses of Cork Harbour, Ireland, captured during the Fenian Uprising of 1867.
Many Martello's survive today, a great many of them intact. Check this list, and see if one is near you - it would make for a unique trip.