The interior is austere and simple. Some of the rough, squared stones, taken from Mount Subasio, were put in place by the saint himself while repairing this little church. It is decorated in a simple Gothic style with frescoes from the 14th and the 15th century. But the masterpiece is the six-part fresco in the apse of this little church, painted by the priest Ilario da Viterbo (1393):