Watercolour of a jersey landmark.
Martello towers are compact, circular defensive forts built primarily in the early 19th century to protect British coastlines from Napoleonic invasion threats.
The design originated from a stubborn Genoese tower on Cape Mortella in Corsica, which British forces captured with difficulty in 1794, inspiring widespread adoption.  
 
These towers typically stand 30–40 feet (9–12 meters) high, with walls up to 13 feet thick at the base, made of brick or stone to resist cannon fire.  They featured a ground floor for storage, a first floor for quarters (one officer and 15–25 men), and a flat roof mounting a 24-pounder gun that could rotate 360 degrees; entry was via a raised door with a ladder or drawbridge.  Some had moats, machicolated platforms for defense, or attached batteries.
 
Jersey’s Martello towers, located in the Channel Islands, consist of eight true examples built by the British to defend against French threats during the Napoleonic era. Above is tower No.2