Orsini's status put him in a position to be a major patron of the arts, and during the pontificate of Martin V (1417–31), the Cardinal of Santa Sabina, as he was called, became the center of an early circle of humanist culture that included Leonardo Bruni, Poggio Bracciolini, Leonardo Dati and Lorenzo Valla, who recalled how the scholars would gather, dressed in antique robes, to discuss topics of human conduct in Classical and Christian terms. Orsini assembled a library with 244 manuscripts, which passed at his death intact to the Vatican Library. His seat was the fortress-palazzo crowning "Monte Giordano", a small rise south-east of the Ponte Sant'Angelo, which had been built in the twelfth century by the Roncioni, and had been converted and extended into a palatial complex by the Orsini.