King of Seas

You have to progress far enough into main story by completing the missions "Power to the Pirates" and this will introduce you to settlement capturing. Fortresses can be difficult targets to defeat, they have a lot of health and powerful defensive canons. The 'meta' for taking fortresses is currently equipping Frigate or a Galleon with the ability "pee on those canons" which reduces the cooldown of canons significantly. Park yourself next to a fortress and activate the ability, spamming the fortresses down quickly with canon fire.

If you find survivability is still an issue, acquire and use the abilities "Ethereal Navigation" or "Seagull Army" to mitigate damage once the cooldown of "Pee on those canons" finishes the global cooldown.

NOTE: This achievement is stackable through multiple playthroughs.
NOTE: The Royale Navy and Merchants Guild will retake settlements slowly over time. You can recapture these for credit towards your total.