Francesco della Rovere came to the papacy from the Franciscan order, where he had been minister general and a working theologian who wrote on the blood of Christ and on the Immaculate Conception. What he is remembered for is not the theology but the building. Between 1473 and 1481 he had the old papal chapel in the Vatican rebuilt to proportions that later scholars noticed match those given for the Temple of Solomon in the book of Kings, and had its walls frescoed by Perugino, Botticelli, Ghirlandaio and Cosimo Rosselli with paired scenes from the lives of Moses and Christ. It has been called the Sistine Chapel after him ever since, and it is where his successors have been elected for five centuries.
He also gave the Vatican Library its institutional life. A papal collection had existed before him, but in 1475 he formally established it with an endowment, rooms and a staff, and appointed the humanist Bartolomeo Platina as its first librarian, a moment Melozzo da Forli painted with the pope enthroned among his nephews. Under Sixtus the collection roughly doubled and became a scholarly resource rather than a private hoard. He rebuilt streets and bridges across Rome, including the Ponte Sisto, in preparation for the jubilee of 1475.
The cost of all this was met partly by an aggressive extension of family power. Six of his nephews were made cardinals, one of them Giuliano della Rovere, who would rule as Julius II and finish much of what his uncle began. Contemporaries objected loudly, and the Florentine crisis of 1478, in which the Pazzi conspirators killed Giuliano de Medici during Mass in the cathedral, drew Sixtus into a war with Florence and left a stain his defenders have argued over ever since.