Heir apparent more or less in open rebellion against his brother King Louis XIII and the Cardinal de Richelieu, Gaston received Blois as part of his appanage in 1626 and began to reside there from 1634. To distract him, « they had set his mind on tearing down the château and rebuilding it », to quote the Memoirs » written by his secretary, Nicolas Goulas.

Gaston chose the Parisian architect, François Mansart (1598-1666) who had already built the châteaux of Berny and Balleroy and had just finished the church of the Visitation in Paris.

After tearing down a wing built at the far end of the courtyard by Louis XII, work on the new main buiding began in 1634 and was interrupted in 1638 in an unfinished state. Initially, it was the beginning of a vast reconstruction of the château that would have entailed demolishing the other wings built by Louis XII and François I. It seems that Gaston’s hopes of mounting the throne were thwarted with the birth of Louis XIV and he preferred to concentrate his financial efforts on Chambord.

Following the failure of the Fronde, Gaston returned definitively to Blois towards 1652-1653. He chose to establish his apartments in the François I wing rather than undertake the completion of his wing which remained a hollow shell up until the beginning of the 19th century when the château became a military barracks. He died there in 1660.

The Gaston d’Orléans wing is a chef d’oeuvre of French classical architecture : the main block with its central pavilllion, the sober and subtle treatment of the façades, the elevation with its three classical orders, the superposition of the triangular and rounded pediments all contribute to accentuate the verticality of the composition. The stylistic counterpoint that this wing offers in comparison to the Louis XII and François I wings has caused divers reactions : Félibien towards 1680 deplored that Mansart’s project had never been completed, while Flaubert, in 1847, saw it as « a building in a sober style which is the inferior style ».

Another masterpiece can be discovered inside : an open cloister vault with carved compartiments representing antique trophies, mannerist masques, and festoons, surmounted by an oval cupola on pendentives abundantly lit by the window openings of the upper story. This sumptuous decoration is attributed to the sculptors Simon Guillain and Michel Anguier.