Headquarters of the VI. Corps of the Imperial Army as well as the Department of Enhanced Inquiry, it currently hosts the 60th Brigade d'Infanterie and the 61st Brigade d'Artillerie. Fort Boyard was originally built when it was found that the swampy Thrice Peninsula was suffused with supernatural energies, possibly from an ancient battle of powerful wizards that was fought here. Certainly that would explain why the region tends to produce so many magic-users!
Fort Boyard lets you recruit Wizards (and Witches!), these existential threats to the Eternal Empire. They cost 10 000 ducats and it takes 1d6 weeks to find a wizard willing to join you.