This rock stands proudly at the mouth of the harbor of this small, remote port town. Due to its isolated nature but valuable trade, this town was the target of coastal raids for centuries and so the king of the region decided to build a fort, carved directly from the rock. A small detachment of naval guardsmen take month long deployments manning this stronghold and heading off pirates and raiders alike. Filled with rations, water, and arms and easily resupplied by sea, this naval bastion remains a tough target for any attacking force.
Why would your party need to visit? Maybe they've allied with the local pirates who need them to clear this fort out to allow them to terrorize these waters unimpeded. Maybe the king has heard tales of a large raiding force and the group is dispatched to guard this rock from waves of attackers. Let me know what your ideas are in the comments below!