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Once you hit level 45 the broadhseet peddlar will offer you the mission Beseiged Tortuga. When you sail to Tortuga and click on it, it will offer you choice between "normal" Tortuga and Beseiged Tortuga.

Beseiged Tortuga is premium content so if you are not a captain club member or have acquired access via burning sea notes this mission is not available to you.

Besieged Tortuga is a group mission so hopefully someone in your group will be able to lead you around.