sentences:  An unusual feature of Montreal's defence was a string of 30 outlying forts to protect against the constant Iroquois threat to the expansion of French settlements.The majority of these were simple stockades, but as artillery was not as developed as on the battlefields of Europe, some of these were built like the fortified manor houses of France.Roughly four of these were substantial stone forts which served as defensive residences, sometimes considered "true castles", as well as imposing structures to prevent Iroquois incursions.