One of the first things you notice about Ibiza Town is the enormous stone fortress rising above the harbour.

These are the Renaissance walls of Dalt Vila, the historic upper town of Eivissa.

Today, they are one of the island's most recognisable landmarks. But they were not built simply to make the city look impressive.

They were designed to protect Ibiza at a time when warfare in the Mediterranean was changing rapidly.

Ibiza had walls long before the Renaissance

The fortress we see today dates mainly from the 16th century, but fortified walls had existed around the upper town for centuries before that.

Dalt Vila occupies a naturally defensible hill overlooking the harbour. Its elevated position made it an important location for settlement and defence over a very long period.

The upper town is more than 2,500 years old, and its present streets preserve traces of its medieval layout as well as much earlier historical phases.

By the medieval period, Ibiza already had substantial defensive structures.

But by the 1500s, these older walls faced a serious problem.

The technology of war had changed.

Why weren't the old walls enough?

The answer was artillery.

During the late Middle Ages and Renaissance, increasingly effective firearms and cannons changed the way fortified cities had to be defended.

Traditional high walls and towers could be vulnerable to artillery fire.

The solution was to build a different kind of fortress.

Instead of relying mainly on tall vertical walls and towers, military engineers developed thick defensive walls combined with projecting structures known as bastions.

These new designs allowed defenders to position artillery more effectively and create overlapping fields of fire.

Ibiza needed this new approach because its strategic position made it vulnerable to attacks from the sea.

Who ordered the new fortifications?

The major reconstruction of Ibiza's defences took place during the reigns of Charles I and Philip II of Spain.

The older medieval fortifications were no longer considered adequate against the weapons of the period, leading to a major renovation and strengthening of the city's defences beginning in the 1550s.

The project formed part of the wider defensive strategy of the Spanish monarchy in the Mediterranean.

Ibiza was strategically positioned between Spain and Italy, and control of the island and its harbour had military importance.

Giovanni Battista Calvi arrives in Ibiza

The first major engineer responsible for the new fortress was the Italian military engineer Giovanni Battista Calvi.

Work on the new fortifications began in the 1550s.

Calvi's initial project incorporated six bastions around the existing medieval perimeter.

His design represented a major change in the city's defensive architecture.

The new fortifications were broader and more sophisticated than the medieval walls they replaced.

Rather than simply making the walls higher, the design focused on creating a system capable of withstanding artillery and allowing defenders to fire along the approaches to the city.

Then the plan changed

The fortress that visitors see today was not completed exactly according to Calvi's original design.

In 1575, the Italian engineer Jacobo Paleazzo Fratin took over the project.

By that time, the city had developed beyond the area originally enclosed by the older fortifications.

A neighbourhood had grown outside the medieval walls on the Puig de Santa Llúcia.

Fratin modified Calvi's original project partly because this expanding urban area also needed protection.

The result was a larger fortified enclosure.

This is one reason the final fortress looks like a carefully planned system rather than a simple replacement wall.

Why are there bastions instead of towers?

The answer comes back to artillery.

A bastion projects outward from the main line of a wall.

This allowed defenders to fire across the faces of neighbouring walls, reducing areas where an attacker could approach without being exposed to defensive fire.

The system was particularly suited to the artillery warfare of the Renaissance.

The seven bastions of Dalt Vila form part of this defensive design.

The walls and bastions were integrated into a single system rather than functioning as isolated towers.

How long did it take to build?

The major Renaissance fortification works took several decades.

The main construction period ran from the 1550s into the 1580s, with Calvi beginning the project and Fratin later expanding and modifying it.

By the end of the main construction period, Ibiza had a substantially modernised defensive system.

Was Dalt Vila attacked by pirates?

Ibiza was certainly exposed to corsair and other maritime attacks during the period when the new fortifications were being developed.

However, it is worth being precise here.

The walls were not built simply because of "pirates" in the popular modern sense.

The defensive situation was part of a much larger Mediterranean conflict involving the Spanish monarchy, the Ottoman Empire, North African corsairs and other powers.

The fortress was therefore designed for the broader military realities of the 16th-century Mediterranean.

The walls protected more than the old town

One of the interesting aspects of Fratin's redesign is that the new fortifications took account of the city's growth.

The objective was not simply to surround the oldest part of Ibiza with stronger walls.

The defensive perimeter was expanded to incorporate areas that had developed outside the medieval enclosure.

This meant that military architecture and urban development were closely connected.

The fortress helped shape the future layout of Ibiza Town.

Dalt Vila became a Renaissance fortress

The finished fortifications were a major example of Renaissance military architecture.

Their design reflected the most advanced defensive thinking of the period, combining thick walls, bastions, gateways and carefully planned lines of fire.

The importance of Dalt Vila extends beyond Ibiza itself. Its fortifications belong to the broader history of Renaissance military engineering and the development of artillery fortresses across Europe and the Mediterranean.

Why are the walls still standing?

Part of the importance of Dalt Vila is that the Renaissance fortifications have survived remarkably well.

The later development of Ibiza did not destroy the basic defensive system.

Instead, the historic walls, streets and buildings remained integrated into the modern city.

In 1999, Dalt Vila became part of the UNESCO World Heritage property "Ibiza, Biodiversity and Culture", recognised as an outstanding example of Renaissance military architecture.

A fortress that changed Ibiza

The walls of Dalt Vila were built because Ibiza had become increasingly vulnerable to the military technology of the Renaissance.

Old medieval defences were no longer enough.

The answer was a completely different type of fortress, designed by Italian engineers and adapted to the changing shape of the city.

The result is the Dalt Vila we know today: a fortified upper town of walls, bastions, gates, steep streets and historic buildings overlooking the harbour.

What looks today like one of Ibiza's most beautiful places was originally a serious piece of military engineering.

And that is perhaps the best way to understand Dalt Vila.

It was not built to decorate Ibiza.

It was built to protect it.