Comagena was a fortified Roman camp on the Danube, on the site of the modern town of Tulln on the Danube in Lower Austria, Austria. Built as a defensive work along the Norican frontier, it originally housed an ala of up to 500 cavalry troops. From its exposed situation, it was able to guard a militarily significant ford over the Danube and control the valley lowlands for a good distance upstream and down.