During the war between Perugia and Arezzo that took place at the end of 1100, the existing Lion Fortress was brought to ruin by Henry IV, Holy Roman Emperor, and during 1297 the Perugia prosecutors decided to fortify the town and to build a fortress inside the castle.
The interventions made possible the realization of the formidable fortress.
During the 1300s new works were taken by Brother Elia Coppi from Cortona and by the Sienese Lorenzo Maitani. Like all  strategically important castles, it suffered several attacks during the fourteenth and fifteenth centuries, until Giampaolo Baglioni established his residence there in 1503, arranging for the restoration and expansion of the complex.
A further stretch of wall, currently still visible, was raised between the Palace and the Keep; It had an access system consisting of wooden stairs resting on the landings that, in case of attack, were withdrawn.
With the additions and the internal system of the Keep, it was considered one of the most difficult European castles to conquer of the 1500s.
In 1550, Pope Julius III promoted the site to a Marquisate and presented it to his sister and his nephew Ascanio (the First) Della Corgna.
In 1554 Ascanio I set up a series of formal gardens within the walls and he replaced two towers with cylindrical battlements, as the round surfaces were more resistant to cannon fire; had his stately residence designed by Vignola, restoring fourteenth-century buildings.
Until the death of Fulvio II , in 1647, the fortress was owned by the same family; then it came under the jurisdiction of the Apostolic Chamber and since 1860 has been owned by the town of Castiglione del Lago.

Bibliography
AA. VV. (1992), Guide De Agostini – UMBRIA, Novara, Istituto Geografico De Agostini
AA.VV. (2004), Umbria - Piccoli Centri della Provincia di Perugia, Cannara (PG), Arti Grafiche Antica Porziuncola
Amoni D. (2000), Castelli Fortezze e Rocche dell'Umbria, Ponte S. Giovanni (PG), Quattroemme