The coastal towers were built during a span of sixty years and can be easily classified into two distinct groups that reflect two separate concepts of coastal defence strategy. The first group were massive squarish towers fitted with heavy pieces of artillery and garrisoned by a sizable detachment of regular troops. These large towers were built not only to guard the major bays susceptible to invasion but were also expected to engage the disembarking enemy with their powerful guns while their garrisons were required to harass and reconnoitre the enemy.

In the period between 1620 and 1649 the Knights had also introduced a much smaller type of coastal tower intended to serve solely as a permanent lookout post rather than as an isolated stronghold. The new watch-towers built during the rule of Grand Master Lascaris, but paid for by the Universita, were intended to fulfil a clear-cut role, enabling the Order's engineers to standardize and perfect their design from the elongated Lippia-type watch-tower to the more squattish example at Wied iz-Zurrieq. The latter was to serve as a blue-print for the chain of thirteen watch-towers built by Grand Master De Redin and designed to relay warning signals all the way to Valletta. The limiting factors that had determined the reduction in the size of the coastal towers and the change in their role were basically ones of manpower - the Order did not have the manpower to post large detachments of troops at every possible landing place. These lessons were soon to be forgotten in the beginning of the 18th century when the Knights again embarked upon the fortification of every bay and inlet around the island, when batteries, redoubts and coastal entrenchments were then the new order of the day.

From
Stephen C Spiteri (1994):
Fortresses of the Cross - Hospitaller Military Architecture (1136-1798)
Heritage Interpretation Services (p.272-273).