Towers that watched the sea.

Why the coast was watched

Formentera sits low and open in the gap between Ibiza and the African coast, and that geography shaped its history. From the sixteenth century onwards, sails on the horizon could mean corsairs, privateers or slavers working the Mediterranean. The island was raided often enough that its population fell away for long periods, and settlement stayed scattered and cautious. Coastal defence was not an abstraction here: it decided where people lived, where they built, and how much they were prepared to leave behind when a ship was sighted.

Raids came without much warning. A ship under a friendly flag could turn; a landing party could be ashore before anyone working in the fields had reached a wall. For an island of scattered farms, thin soil and few horses, the sensible answer was to see far and early, then move people and animals somewhere defensible. The towers were built to buy time, not to fight, and nobody expected them to do more than that.

A chain around the islands

Formentera was never defended alone. The Pitiusas, Ibiza and Formentera together, sat on the sea roads between the Iberian coast, North Africa and the rest of the archipelago, and the whole group organised its coast in the same way. A lookout on the south of Ibiza could see across the channel; a signal could reach Formentera and be passed on again. The system mattered more than any single building. Most towers were small, and their value lay in being part of a line.

The same logic ran through the islands: watch the sea, pass the warning, move inland. Formentera's towers share the family resemblance of Mediterranean coastal defence, being square or round, slightly tapering, built of the local stone, with the door raised above ground level and a flat platform on top. Few were garrisoned for long. In quiet years they were maintained badly, and many lost roofs, floors and stairs long before they lost their walls.

How a watchtower worked

A tower was a workplace rather than a monument. A small garrison, often only a handful of men, kept a store of water, dried food, firewood and powder, and took turns at the top. The watch was kept in daylight and at night, and the job was dull until it was not. A tower that saw nothing for a month still had to be manned, and the cost of that patience fell on the island itself.

The warning was simple. Smoke by day, fire by night, and a rough code of signals that a lookout on the next headland could read: one fire for a distant sail, more for a fleet, a shot for a landing. Messages moved faster than any rider. What the tower could not do was stop a landing, so the next step belonged to the church and the walled refuges inland, which is where the population went.

• The watchLookouts kept station on the platform, day and night, in all weather.
• The signalsSmoke by day and fire by night carried the warning along the coast.
• The storesWater, food and firewood were kept in the tower against a long wait.
• The limitA tower could warn but not fight; the real defence was inland.

Stone, lime and wind

Formentera's towers are built from what the island has: limestone, cut roughly and laid with lime mortar, with rubble in the core. The stone is soft enough to shape and hard enough to stand for centuries once it is kept dry. What damages a tower is not the wind alone but water getting into a wall, then roots, then a fall of masonry. A tower with no roof and no mortar in its joints is a tower in the process of becoming a pile.

Repair is slow, specialised work, and much of it has happened only where a tower stands near a path or a car park. Elsewhere the walls are held by vegetation and luck. This is why the most useful thing a visitor can do is also the least dramatic: stay off the walls, do not pull stones loose, and do not add cairns or marks. The ruin you see is the whole of the evidence, and it does not renew itself.

The church as a refuge

The tower was only the first half of the system. The second was a place to go. In Formentera that place was the church of Sant Francesc Xavier, built in the eighteenth century as a fortified refuge rather than only a place of worship. Thick walls, small windows, a tower and a well inside made it somewhere a scattered rural population could hold out with animals and whatever food could be carried there in a hurry.

The arrangement tells you how the island understood danger. Farms were worked, but the church was where life was kept. Similar fortified churches stand on Ibiza, and the pattern is Mediterranean rather than local: when the coast is raided, the inland village becomes the safe house, and religion and defence share a single building. The tower of Sant Francesc still reads as a watch post as much as a bell tower.

Towers you can walk to

Nothing on Formentera is far. The island is about 19 km long, and the coast roads and signposted green routes pass within reach of most of the old lookout points. Walking is the best way to understand the siting: you climb a little, the land opens, and you see the channel to Ibiza and the water where a sail would have mattered. Several of these points are marked on the green routes for walking and cycling.

Not every tower is easy to reach. Some stand on private land or on islets, and a few are no more than a shape in the scrub. The lighthouses at Cap de Barbaria and on La Mola occupy headlands that once did the same job and are simple to visit. Treat the towers themselves as things to look at from outside unless a clear path and a sign invite you closer.

• Sant FrancescThe fortified church and its tower stand in the middle of the island's capital.
• Cap de BarbariaThe southern cape has a lighthouse and long views back across the island.
• La MolaThe eastern plateau and its lighthouse mark the old lookout line to the east.
• EspalmadorThe islet north of Ses Illetes keeps a small tower, best seen from the water.

What the towers look at now

The view from a lookout point today is not empty, but the ships are ferries, freight and yachts, and the danger has changed. The water between Ibiza and Formentera holds the posidonia meadows that form part of the UNESCO World Heritage listing of 1999, which also covers Dalt Vila, Sa Caleta and Puig des Molins on Ibiza. From a headland you can see the colour the meadows make: pale turquoise over sand, then the deeper green of the plant.

The towers themselves have been out of work for a long time. Lighthouses took over the signalling, radar took over the watching, and the last useful thing a tower did was to stand still while everything around it changed. That is roughly what a visitor sees now: a small building on a headland, doing nothing, explaining a great deal. Late afternoon suits them, when the stone takes the light and the sea goes flat.

A tower is only a wall with a view until you remember what the view was for.

Seeing them sensibly

Most of the coast is open to walkers, and the towers are small enough that a visit takes minutes. The difficulty is the setting. Formentera is flat, dry and short of shade, and the paths to headlands run over limestone and juniper with little cover. Water, a hat and shoes with a proper sole are more useful than any guidebook. In summer, walk early or late and give the middle of the day a miss.

Out of season the island is quiet. Many businesses close roughly from November to April, but the paths, the headlands and the towers stay where they are, and winter light is clear and low. Ferries from Ibiza are fewer then, and the Consell de Formentera's high-season limits on visitor vehicles change how you travel in summer. Check the current ferry timetable and the vehicle rules before booking anything.

• WaterCarry more than you think you need; the headlands have no shade.
• FootwearThe limestone is sharp and uneven, and a thin sole is a mistake.
• TimingWalk early or late in summer, and avoid the strongest midday heat.