On the kingdom islands you can find new shops where you can buy cannons and cannonballs. You need to place the cannon on overworld (sand or grass) and feed it with some cannon balls, then grab it and aim. It can destroy ships and fortifications and is a good tool to defend your islands.
The former trading islands will soon be changed to be conquerable, so you can build a fort on it and defend it, protected with some rules to prevent being overtaken when the kingdom members are asleep.