(CNN) — They're scattered all over Europe but their perfect geometrical beauty can only be fully admired when seen from above.
Like snowflakes viewed through a magnifying glass, there's a dazzling fractal-like quality to the neat, intricate way they've been laid out -- just that we're not talking here about natural wonders.
An entirely new, rational way to design fortified settlements, the "star cities" of the 16th and 17th centuries were a jewel of the early modern era and engineered by some of the most brilliant minds of their time.
The science of building star-shaped bastion forts came into being in Italy during the Renaissance, when gunpowder and cannon rendered old medieval walls obsolete.
High, vertical walls gave way to low-lying ramparts, which offered less of a target, while wide moats, sloping earthworks and complex networks of protruding bastions would eliminate any blind spots and prevent besieging armies from coming anywhere near the ramparts.
This style of military architecture would have its golden era during the 1500s and particularly the 1600s, a period in which many parts of Europe saw almost uninterrupted warfare.
It's not a coincidence that some of its best and most magnificent examples cluster in places such as the Netherlands and the Rhine Valley, fault lines between warring empires.
Military engineers such as the Frenchman Sébastien Le Prestre de Vauban and his Dutch rival Menno van Coehoorn elevated the fortification of cities almost to the category of art.
Successive changes in military technology rendered many of their "star cities" obsolete, but since many of them had by then become civilian settlements, this eye-catching approach to urban planning was not lost to time.
What follows is a selection of some of Europe's most beautiful star cities. While the list of star-shaped forts in Europe is much longer, we have deliberately focused on those that became home to living communities, all while preserving the original geometrical layout.
The Netherlands is a prime destination for those interested in "star city" architecture. The Dutch Revolt (1566-1648) is the name given to the long years of war that followed the Dutch rebellion against Spanish rule and it left its mark on the urban landscape of a land otherwise devoid of natural fortifications.
Perhaps the most impressive of the Dutch star cities is Naarden, some 20 kilometers east of Amsterdam. Entirely surrounded by two lines of fortification and two rings of water, Naarden might just be the perfect star-shaped city. It is also, very fittingly, the site of the Dutch fortress museum.
Brielle is a seaport once seized by the Watergeuzen (or "Sea Beggars").
The city of Brielle has a place of honor in Dutch history since 1572 when its capture by the "Sea Beggars," a seafaring nationalist militia, marked a major turning point in the Dutch war of independence.
Although now dwarfed by the docks of neighboring Rotterdam Europoort, Europe's largest commercial port, its neatly laid out bastions and moat are easy to spot from the sky.
Completed in 1597, Heusden can boast today of its impressive, neatly arranged bastions thanks to an award-winning, 40-year-long reconstruction project.
By the 19th century, this North Brabant town's original fortifications had all but crumbled away. However, starting in the 1960s, local residents led a successful campaign to restore Heusden to its former glory. A 300-year-old map was used as a blueprint to turn the place into a beautiful period town that pre-pandemic was getting hundreds of thousands of visitors every year.
Also in the region of North Brabant, this small town is another prime example of a Dutch star city.
It's said that each of its seven bastions represents one of the seven provinces that united to liberate the Netherlands from Spanish rule in the 16th century, forming an independent Dutch state.
In the northeast Netherlands, just yards from the German border, Fort Bourtange was built in 1593 and used until 1851.
The fortress was then turned into a village, preserving its perfectly pentagonal shape and geometrical street pattern, but it never really prospered as a civilian settlement. Today, the whole architectural ensemble is open to visitors as a museum.
Palmanova was founded in 1593.
Located very close to the present-day border between Italy and Slovenia, Palmanova (not to be mistaken for the Majorcan resort of the same name) is the quintessential geometrical city as well as one of the largest and best preserved. The Venetians built it in the late 16th century in order to defend the northeastern border of their Most Serene Republic.
Its radial layout expands from a central hexagonal square (the "Piazza del Duomo") in concentric nine-sided rings traversed by straight avenues leading to each of its angles. The whole ensemble is encircled by an outer double perimeter of star-shaped fortifications.
Palmanova has been designated a World Heritage site by UNESCO (together with other Venetian fortifications of the same era).
Peschiera del Garda
If Palmanova protected the eastern approaches to Venice, its western ones were under the watch of Peschiera del Garda, which also has World Heritage Site status. Quite appropriately for a place that was long ruled by Venetians, the fortified city is not so much on the shores of Lake Garda, but inside of it, surrounded by water on all its sides, with a canal cutting right through it.
Almeida is now a small town of around 1,300 inhabitants.
In northern Portugal, very close to the Spanish border, this town fits perfectly inside its imposing star-shaped ramparts.
During the Peninsular War (1804-17), it was captured by Napoleonic forces under French military commander Michel Ney (himself born in another star city, Germany's Saarlouis) after a gunpowder magazine exploded, killing hundreds of its British and Portuguese defenders.
Around 200 kilometers from Lisbon, Elvas is a stronghold of seven bastions and two forts that for centuries guarded Portugal's eastern border against Spanish incursions.
Although its urban grid lacks the cold, geometrical lines of other star cities, its whitewashed houses and hilltop location make Elvas a picturesque sight.
Here is another star city that enjoys World Heritage site status, as part of the network of fortresses created by Sébastien Le Prestre de Vauban, Louis XIV's greatest military engineer. It was built from scratch on the French side of the Rhine, after France lost the city of Breisach on the opposite bank of the river, hence the prefix "Neuf" (New).
It was conceived from the start for mixed civilian-military use, following contemporary notions of what an "ideal city" should be like, with an octagonal design and streets arranged in a square grid.
This military fortress has been a prison and a concentration camp.
Built in the late 18th century as a garrison town by Habsburg emperor Joseph II, who named it after his mother, Empress Maria Theresa, Terezín has seen little in the way of frontline fighting.
During World War I, the fortress was used as a political prison camp. Gavrilo Princip, the man who helped trigger the war by assassinating Archduke Franz Ferdinand, died here in 1918 of tuberculosis.
After the Nazi takeover of Czechoslovakia, the Gestapo used part of its facilities as a prison and the whole town was later turned into a concentration camp and ghetto. Jewish prisoners from all over Europe were crammed between its walls, many of whom were later shipped to extermination camps. It is estimated that of nearly 150,000 people who went through what the Nazis called "Theresienstadt Ghetto" during the war, only some 23,000 survived. Today it is the site of the Terezín Ghetto Museum.
A UNESCO World Heritage site, the old town of Zamosc has retained most of its original 16th-century layout.
A big part of its uniqueness rests on the fact that it mixes architectural influences from different parts of Europe, since Zamosc was a trading outpost that attracted merchants from east and west. Also, the town's plan was designed by an Italian architect from Padua who incorporated architectural elements from his native land.
This hexagonal star city was built by the Habsburgs in 1579 as a bastion to protect their lands against the Ottomans. In the century that followed the latter laid siege to it up to seven times, each one of them, unsuccessful.
Although modern suburbs have developed all around Karlovac's old town, the harmonious architectural ensemble at its core has been well preserved.