The Church of St. Sisto is outside of the town of Onelli close to the ancient castle walls, at the junction of the road for Cascia.

A renaissance portico was laid against the 14th century building and its arches were later closed. The ogival portal bears an Agnus Dei on the keystone and a coat of arms belonging to a noble marquis.

The large interior has a stone floor with tombs, a pulpit for Lenten sermons and was vault-covered; it is embellished by seven wood altars of the 17th and 18th centuries (High, Rosary, Conception, Crucified, St. Michael Archangel, Annunciation, Madonna of Carmel) with canvases of the same period: Madonna with Saints Anthony of Padua, Michael and Sisto; Annunciation (copy by C. Maratta); Madonna of the Rosary (by Sebastiano Conca); St. Sisto brought to martyrdom; Oration in the garden (first altar on the right, of Roman art); Immaculate Conception (first altar on the left).

The organ, which is still operational, is of the 18th century.