The Rocca Monaldeschi della Cervara is a thirteenth-century fortress that dominates the hilltop medieval quarter above the lake shore. The castle was built to control the Via Cassia, the ancient Roman road running north-south through Lazio, and served as the seat of local power through the medieval period. Today it houses the Museo Territoriale del Lago di Bolsena.
The museum covers the natural history of the volcanic lake and the archaeology of the surrounding area, with exhibits on the Etruscans, the Roman settlement, and the geology of the volcanic crater that created the lake. The views from the castle walls over the lake and the two islands — Bisentina and Martana — are excellent. Entry is inexpensive and the combination of the castle architecture, the museum, and the views makes it worth the short climb from the town centre.