This is a small idea that I haven't given much thought to but it exists as is to be built upon or destroyed, but hopefully, it sparks thought.
A Floating fortress of skeleton ships, it is essentially a fortress built of ships connected together by bridges, planks, and tethers, it is a fortress complete with large crows nests, cannons, and other accouterments for the defense to go along with the regular cannons on the many ships that make up its contents.
The Floating Fortress will sail on a predetermined path for the duration of the event and leaves once it reaches the end, The Floating fortress can be attacked from your ship and boarded, have its masts broken, lit on fire, and any other regular elements of ship combat but will not sink until it is completed. The captain's quarters on each linking ship are locked holding the valuables you will want once the fortress is destroyed. There will be some other valuables on board to grab for a few quick picks.
The ship will have named skeletons with a myriad of skulls to collect on the journey for players who really want to board the massive fortresses and take the skeletons head-on.
So there should be enough treasure for each of the different shipping companies to feel rewarded for taking on such a feat.
The rear and the front side of the Fortress are vulnerable but still guarded by cannons from up above in the crow's nests bridges and rampways that lead up to and across the fortress.
This is the current idea as it stands, but I believe the thought of a floating fortress available to take on during event times with other crews on the servers would be a fun experience and a test of the alliance system and individual pirate behaviors. As the Fortress should provide enough difficulty for 2 or 3 full crews whether piloting, gunning, or swashbuckling.
C.O.S.
