Martello towers (or simply Martellos) are small defensive forts built by 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 25 men. Their round structure and thick walls of solid masonry made them very 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 being preserved as historic monuments.