Isabella d'Este (1474-1539) (Wikipedia), one of the most famous women of Renaissance Italy, was not only a patron of the arts but took control of the government of Mantua during the absence of her husband, Francesco Gonzaga, Marquess of Mantua. In particular, the marquess was captured in August 1509 when laying a siege to Padua during the War of the League of Cambrai (Wikipedia) and was held prisoner by the Venetians until July 1510. Isabella managed to win support of the pope Julius II and obtain the release of her husband, while fending off intervention of the French King and the Holy Roman Emperor (Shiono Nanami, Women of Renaissance (in Japanese)).
Isabella used cipher throughout her political career (Cockram n.17 quotes her reference to cipher in 1498). Some of her ciphers are preserved in the Archivio Gonzaga (AG) (Wikipedia) in Archivio di Stato di Mantova (ASMn), containing various ciphers between 1395 and 1702 in bb.423-425 (Cockram n.18).
Two letters in cipher addressed to Isabella d'Este during the captivity of her husband in Venice are reproduced in Cockram (2009). Both relate to negotiations for the release of Isabella's husband by offering her son as a hostage to either the Emperor, the French King, or the Pope.
The first example is a letter from Donato de Preti, Gonzaga's envoy at the Imperial court, to Isabella dated 9 March 1510. The simple cipher can be reconstructed as follows.
The second is a letter from Jacopo d'Atri, Isabella's representative at the French court, to Isabella dated 6 June 1510. The cipher is used in a postscript on a separate sheet. Cockram (2009) points out such a postscript allows the recipient to show the main body of the letter to others at court, while the true, confidential intention was conveyed in the postscript. The cipher, reconstructed below, is slightly more complex than the above with a null and homophones. Codenames are also employed such as Solis (the King of France), Bifores (the Emperor) (these two can be seen in FIGURE 4 in Cockram (2009)), Materiam (the King of Aragon), Argenti (the Pope), Cignentia (Francesco Gonzaga), and Medias (the Duke of Ferrara). An original copy of this cipher is preserved in the archives.
Cockram (2009) describes other examples of Isabella's counter-intelligence measures, including sending completely fake letters.
Sarah Cockram, "Epistolary Masks: Self-Presentation and Dissimulation in the Letters of Isabella d'Este", Italian Studies, Vol.64, no.1, Spring, 2009, 20-37 (Academia.edu)