Formidable fighters. They love to fight for the highest bidder!
They are usually second-choice to their more powerful, albeit more expensive counterparts: Boatswains and Buccaneers, however, they can do their job rather well nonetheless. Just make sure you pay attention to their health, and retreat when necessary. It is advised to back up your Corsairs with Assassins and Templars, so as to stall enemies while the Corsairs heal.
As with all soldiers, build support towers.