The city of Zamosc, Poland, "one of the most authentic Renaissance cities left in Europe," was "designed in 1578-1580 by Italian architect Bernardo Morando of Padua," Italy (Historic Cities). What you see in this beautiful 1617 Braun & Hogenburg map is the Old Town, surrounded by a star fort, also known as a trace italienne. It was the latest in fortification technology in the fifteenth and sixteenth centuries, developed to defend against the newly developed canons, which could easily level a medieval castle:
[Star forts] were built of many materials, usually earth and brick , as brick does not shatter on impact from a cannonball like stone does. Another important design modification was the bastions that characterized the new fortresses. In order to improve the defense of the fortress, covering fire had to be provided, often from multiple angles. The result was the development of "star"-shaped fortresses. (Wikipedia)
According to Geoffrey Parker in his article "The military revolution 1560–1660: a myth?", the appearance of the trace italienne in early modern Europe, and the difficulty of taking such fortifications, resulted in a profound change in military strategy. "Wars became a series of protracted sieges", Parker suggests, and open-pitch battles became "irrelevant" in regions where the trace italienne existed. Ultimately, Parker argues, "military geography", in other words the existence or absence of the trace italienne in a given area, shaped military strategy in the early modern period. (Wikipedia)
One of the most elaborate star forts was constructed in Bourtange, Netherlands, in the sixteenth century.
The popularity of star forts did not long outlast the nineteenth-century invention of the exploding shell -- although there is a nineteenth-century star fort in Japan, constructed during the Tokugawa shogunate.