martello tower

The Irish coastline is dotted with the famous Martello Towers, small round towers with flat roofs. These towers were designed and built as lookout posts during the Napoleonic Wars.
Should an invading ship be spotted by one of the lookouts in the tower, he would make his way to the roof and set fire to a bundle of wood.

This signal would eventually be seen by the next tower location, who in turn would do the same until all the towers along the coast would be notified of an approaching unidentified vessel.
The Martello tower in this painting in Howth was transformed in the late 1800s to house the first trans-channel cable between England and Ireland.