When your ship docks at an island, on the next turn, place this fort, it now acts as a second home island.
It sounds like the Nexus Tower is mandatory to build? Ex: Once you have at least 30 gold on your HI, you would have to build it at the next opportunity after docking.
I can’t quite tell if it’s supposed to get around the regular fort rules where a ship has to be docked there to build it. Does the ship in question have to stay at the island on the following turn, or can it leave and the fort is still built at the end of your next turn anyway?