On the foothills of the Sabine mountains, overlooking the countryside that stretches all the way to Rome, a solitary tower stands guard above the hamlet of Catino, which is part of the town of Poggio Catino.

Poggio Catino is named after the enormous karstic sinkhole that hides beneath the hamlet and has helped make the site a formidable fortress. In the early Middle Ages, the area marked the border between the Roman/Byzantine territory of Rome and the Lombard duchy of Spoleto, and one of the main thoroughfares through the mountains.

The ruins of the fortress, which is mentioned in the work of the town's most famous citizen, Gregorio da Catino, in his Regestum Farfense, are perched on an outcrop above the hamlet and surround a central keep, with an enormous pentagonal tower (108 feet tall) and that seems unassailable. It's also survived earthquakes of the past few centuries.

That may not have been by accident. To make the tower lighter and more resilient, the Lombard lords had the upper section built of spongolite, a stone made from fossilized sponges, which could be extracted nearby.