For their formal sessions ("sessiones"), the participants of the Council met at the impressive cathedral. Yet, very few debates actually took place in the former episcopal church. Topics were discussed in advance by the Council nations and, as a rule, unanimously approved.

Pope John XXIII opened the Council of Constance on November 5, 1414; it ended on April 22, 1418. In between, altogether 45 sessions took place at the cathedral, which was specially reconstructed for the event. During liturgical services, crucial decisions were arrived upon. Two decrees proved to be groundbreaking for the progress of the Council and the destinies of the Church: "Haec sancta" placed the Council above the pope. "Frequens" was a resolution to regularly hold councils in the future.

The people of Constance did not get to see much of the sessions at the cathedral. Yet, the large number of processions, festive entries, or the Christmas Eve service with king and pope in 1414, gave them an idea of the momentousness of the assembly in their hometown.