The lighthouse on Cardiff Hill above the town is not, despite the name, a working aid to navigation — it was built in 1935 to mark the 100th anniversary of Mark Twain's birth, dedicated by President Roosevelt over a telephone link, and rebuilt after a fire in 1960.
The climb up the 244 steps is steep but the reward is a fine view over the Mississippi, the bridge across to Illinois, and the bend in the river that brought paddle steamers up to Hannibal in Twain's day. Sunset is the time to go.