Lighthouse (Civ4)/Civilopedia

A lighthouse is a tower constructed on the shoreline, usually near a harbor or busy waterway. A powerful light sits atop the tower, making the structure visible to mariners for miles around. Lighthouses are built to mark the location of rocks and shoals or other hazards in the dead of night or midst of storms. Before the invention of radar, electricity, and global positioning systems, any vessel sailing near the shore ran a risk of running into such hazards and sinking. Naturally, sailors preferred to sail to destinations that had a method of marking these perils, so harbors with lighthouses flourished.
The original "lighthouse" was probably a bonfire lit by villagers to guide the village's fishing vessels back to harbor at night. Over time people realized that the fires could be seen much farther away if they were built atop platforms or towers. Further improvements were made as technology advanced; mirrors were built to direct the light in the desired directions; oil lamps replaced the more dangerous bonfires, which were in turn replaced by electric lamps.