Just beyond the fort is a turret that can take down those battleships.
Get to the Turret is a mission in Ratchet & Clank (2016 game) taking place in Fort Krontos on planet Batalia. After Ratchet and Clank fought through with Cora Veralux to travel to the Fort, they found it inaccessible due to laser turrets blocking the immediate entrance to the port. Cora then noted they needed to find another way inside, using the taxi.
The Magneboots allow you to travel up the magnetic path, with unrestricted movement, though you are only able to use the OmniWrench. Travel through here, jumping over hazards and using the wrench to destroy versa-mines that appear, and you will arrive inside Fort Krontos. Here, destroy the bladeballs, blarg generator, and the warbots, then take the path up to the right to reach a bolt crank, extending the bridge while disabling the turrets and allowing Cora to cross.
After this, several warbots and blarg bombthrowers will arrive. As you head further into the fort, two cannonball tanks will appear. Destroy both, and the remaining enemies, to find a bolt crank in front of a large door leading to the exit and towards the Starwatch Cannon. Instead, head left first, to a group of crates; instead of breaking them, jump on them to head up to the upper level, then head left to find a gold bolt perched on top here. Grab it and head back down, then use the bolt crank to open the door. This will also extend a bridge on the other side leading to the Starwatch Cannon.
When you reach the Starwatch Cannon, the mission "Destroy the battleships" will commence.