It seemed like every walk we took on Malta contained a watchtower from the period of the Great Seige of 1565.
Here's a useful summary of their development displayed in one of the towers
Several towers are opened to the public and are manned by enthusiatic volunteers. Just up the road from our hotel was one of the more impressive ones, the Red or St Agatha's tower (link).
Tremendous views from the top
A more typical example
Some more details on the towers from the Military Architecture site:
Wignacourt’s Bastioned Towers
Għajn Ħadid Tower and Aħrax Tower
A 3D reconstruction of the Comino tower on the national Trust of Malta site