The initial towers of this type were built under the auspices of Grand Master Fra Aloph de Wignacourt. A total of seven towers of this type were constructed, and five remain.
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Unlike the later Lascaris towers and De Redin towers, the Wignacourt towers were more than simply watchtowers. Instead they formed significant strongpoints intended to protect vulnerable sections of the coast from attack.
http://en.wikipedia.org/wiki/Wignacourt_towers