Serranilla is the easiest of the top tier forts and it is commonly under attack by Spanish ships that make great healing for the Jackdaw if you get in trouble. Though its firepower and 7 defense is worth mentioning you can always sail out of range to the north or south of the fort to recover a bit and allow you to turn around safely out of range of the cannons and mortars.
Once you are on the ground, enter the fort and cross the wire to climb the wall on the other side, following the roofing to an area of the wall that you can climb. Sneak up on the first Captain and kill him and then take out the second Captain to the east with a Berserk Dart and let your enemies take care of him. Turn west and head up the ramp to the final Captain and finish him off so you can approach the war room and kill the fort's Commander.
After the fort is cleared you can get the Treasure x2 and Animus Fragment x2, and start the Naval Contract, The Final Contract.
