Lighthouse is a buildable structure near the far side dock.
A lighthouse allows for a safe return to the island it's built on without the boat being destroyed.
Exception: if the Monarch loses the Crown, the boat will break, even if there is a lighthouse on the first island—that is where heirs start their journey.
The structure is built and upgraded instantly and without the need of builders.
After that, the lighthouse can be upgraded to stone for twelve additional coins, and finally to iron for eighteen additional coins.
The purposes of the lighthouse upgrades are unconfirmed. For the speculations, see: Lighthouse tiers.
When arriving at an island with a lighthouse of any tier, the boat will arrive, intact, at the far side dock.