'Star forts', 'gunpowder fortifications', and the 'trace italienne'

Image 1. Forte Michelangelo, Civitavecchia, Roma (1508-1535). The fort was commissioned by Pope Julius II (1443-1513) to the Renaissance architect Donato Bramante (1444-1514) to defend the port against pirate incursions. The structure was finished by Bramante's pupils Giulio Leno and Antonio Sangallo the Younger. It is believed that the taller octagonal tower (Maschio) was added by Michelangelo later. Despite the participation of several famous Renaissance architects, Forte Michelangelo is still a traditional medieval fortification with round towers that have blind spots. (Georgiana Hedesan)

Image 2. Citadel of Jaca, Spain. This fort at Jaca, the capital of Aragon, was commissioned by Philip II of Spain in the 16th century. It was the first citadel in Spain to be built to defend against cannon-fire with low thick walls.

Further resources. Innumerable fortifications of this type spread across Europe in the sixteenth, seventeenth and early eighteenth centuries, and around the world during the period of European expansion and colonisation. Listings and images aplenty can be found on Wikipedia and elsewhere, including Architectural Digest and enthusiasts' websites such as The Starfort Gallery and starforts.com.