An island needs only a completed fort to begin construction of an inn.
Use
Inns have historically been a gathering place for carousing and multiplayer puzzles. Each inn has an old salt, who, when well sauced, speaks of goodly shipments. Inns are often highly crowded and serve as marketplaces for tradeable items like clothing and ships, although spamming should be avoided. The deedholder of an inn does not change with the governorship, and the inn has no taxes due and collects no profit.
Owner transfer
On the Obsidian Ocean, inn ownership transfers normally to every flag that wins a blockade. On every other ocean, the inn deed is never transferred except through player trades.