River Castle
The "castle" in the middle of the Mississippi River is not really a castle, of course. It is a water intake facility for the Chain of Rocks water treatment plant, which opened in 1894 and is still open today. It stands in the middle of the river just south of the Old Chain of Rocks Bridge, which is famous enough as part of Route 66, now open to pedestrians and bicycles only. You might recognize it as the location where the final scenes of ESCAPE FROM NEW YORK were filmed.