This is an amazing example of a “fort.” My forts, and the forts I build for my son, with their pillow walls, and slumping blanket roofs, pale in comparison. Click through to the source link for more images of this pirate ship fort.
Pirate ships were early sailing vessels called Square Rigged Ships. These were characterized by driving sails that were large rectangular sheets hung from horizontal beams of wood. Compared to modern sailing vessels these were slow and cumbersome, but they did have planks to walk, decks to swap and timbers to shiver me.
The word Fort is short for Fortification, or a military building deigned to be defensible in times of war or battles. Early fortifications were larger earth or stone walls built around cities, or city centers to protect against invasions. A good fort must also have walls to protect against invading sisters. A good strategy, employed by Nebuchadnezzar at the Ishtar Gate of Babylon, is to have relatively small, strong gate with limited aproach. Otherwise the gate could be overrun with girls.
Many Castles in Europe have moats, usually filled with legendary monsters of the water. The moats prevent sisters from approaching with ladders or siege towers that allow them to breach the outer wall of the fortification. They also prevent invading armies of girls from digging under the walls and undermining the foundation.