She was married at the age of ten to Girolamo Riario, but the marriage was likely not consummated until she reached the "legal age" of 14. Girolamo was the nephew of Pope Sixtus IV, and the young couple lived in Rome in service of the pope. Catherine gave birth to the first of many children at the age of 15. However, the lives of Girolamo and Catherine were to change abruptly upon the death of Sixtus IV on August 12, 1484.
As was the custom at the time, upon the death of the pope, the city of Rome erupted into chaos. The Palazzo Orsini, in the Campo di Fiori, where Girolamo and Catherine had been living, was stormed and stripped of its furnishings. Catherine, on behalf of her husband (and seven months pregnant at the time), rode on horseback across the Tiber, and took possession of the rocca (fortress) of Castel Sant' Angelo, and effectively prevented the conclave from forming, as the cardinals were afraid for their lives.
Caterina remained in control of Castel Sant' Angelo for almost two and a half months, sometimes in opposition to her husband. Finally, she was persuaded to abandon her post by the enticement of 8000 ducats and the reinstatement of ownership of Forli and Imola, which had been granted Girolamo Riario when his uncle was pope.
I first encountered Caterina Sforza in the excellent book, The Tigress of Forli, by Elizabeth Lev. Recently, I watched The Borgias (a Showtime series with mixed reviews), where the part of Caterina Sforza was played by Gina McKee, a talented English actress. The portrait above is by Lorenzo di Credi; it hangs in the Pinoteca of Forli.