The name clearly derives from the presence in the town of the Castle Torriani - Visconti and of the river Ticino in the nearby. Town in the province of Novara, situated between Lake Maggiore and the Ticino, on the border with Lombardy, the municipality is divided into a number of districts and characterized by the presence of rivers, streams, forests and areas ideal for trekking and cycling; Castelletto is also connected by a bicycle trail to Varallo Pombia. Another highlight of the area is the botanical trail recently opened.
The first settlement is of the Bronze Age, witnessed and documented by the presence in the area of populations of Celtic origins. The particular location, close to shores of the lake and the river, is protected by the surrounding hills, and has encouraged the formation of several settlements. In a short time the main center became an important commercial and trading hub, but in the V century B.C. the area recorded a decline of the population growth due to the creation of the nearby city of Milan. In 338 B.C. here settled the Gauls, followed by the Romans. In the medieval ages the town was granted in fief to the Torriani at first, then to the Counts of Pombia, the Casati and finally to the Visconti and was annexed to the Duchy of Milan. Since 1340 Castelletto acquired a certain autonomy and controlled a large amount of the commercial flows through the area. In the XVIII century the town became a possession of the Savoy, except for the short period of the Napoleonic dominion.

Not to miss:
- the Church of Saint Anthony, built in 1775 on the site, where once stood a previous XV century church. It presents a façade composed of two floors and enriched with terracotta statues erected in the late XIX century by Marietti. Inside it preserves two XVIII century paintings by an unknown author. It is flanked by a Bell Tower, built in the first half of the XIX century, designed by the architect Molli;
- the Church of Santa Maria d'Egro, whose presence is already documented in the first half of the XIV century and was rebuilt in the Baroque period. It features a rectangular plan with three naves and a presbytery;
- the Oratory of San Carlo, today desecrated, built in 1750 in late Baroque style and features a single nave plan;
- the Oratory of St. Hippolytus in the village of Glisente, which features a single nave plan and inside preserves frescoes from the XV and XVI centuries;
- St. Anne's Church, inside which are preserved frescoes of the XV century;
- the Castle, built by the Della Torre family, it became residence of the Visconti family, whose descendants still live here. It still features two of the original four square stone towers;
- the Archaeological Park of Sibilia.