Osterio Magno, known as Domus Regia and the first residence of King Roger II, was built in the 13th century by the noble Ventimiglia del Maro family, marquises of Geraci and princes of Castelbuono. Used as a winter residence, it comprised several structures and gardens covering a large area.

A 16th-century drawing shows the Osterio Magno with its tall tower, located at the corner of Corso Ruggero and Via G. Amendola. The surrounding area included factories and open spaces, with a central well and a large archway connecting the complex to the buildings opposite, where the kitchens and service rooms were located. Sold to the Ventimiglias until 1599, it then passed to Simone da Fiore and later to the friars of the Convent of San Domenico. The palace was later adapted into dwellings, workshops, warehouses and even a prison.

The Osterio Piccolo building, with its mullioned tower, is now incorporated into the bell tower of the Church of Maria SS. Annunziata in Corso Ruggero. During a restoration in 1988, Prof. Amedeo Tullio discovered a Hellenistic-Roman residential complex under the Osterio Magno, with finds including a large jar with bronze coins from the 4th century B.C. and a cistern still visible in the ground floor room. The walls at the base of the corner tower date back to Norman times.

The oldest core, known as the ‘bichrome’ building, is characterised by horizontal bands of tufa and lava stone, with elegant double lancet windows visible from Via G. Amendola. The tower was elevated in the first decades of the 14th century, developing over three storeys with swallow-tailed battlements, which no longer exist.

The tour of the Osterio Magno is managed by the Cooperativa Sociale Il Segno and is included in the Cefalù Cathedral Itineraries. Itinerari della Cattedrale di Cefalù.