From the Catholic Encyclopedia:
When, in 1234, the army of Frederick II was devastating the valley of Spoleto, the soldiers, preparatory to an assault upon Assisi, scaled the walls of San Damiano by night, spreading terror among the community. Clare, calmly rising from her sick bed, and taking the ciborium from the little chapel adjoining her cell, proceeded to face the invaders at an open window against which they had already placed a ladder. It is related that, as she raised the Blessed Sacrament on high, the soldiers who were about to enter the monastery fell backward as if dazzled, and the others who were ready to follow them took flight. It is with reference to this incident that St. Clare is generally represented in art bearing a ciborium.