Okay, this is not exactly a secret place but when I ask people (Parisians and tourists) most have never heard of it. The first time I saw this horloge by Jacques Monestier, sculpteur d'automates, Le Défenseur du Temps was fully functional and just amazing.
You can find the whole story in the imbeded links but here is the Genie Notes version: The clock was commissioned in 1975 and installed in 1979 in le Quartier de l'Horloge, so named for this work. At the striking of each hour the man with his sword and shield fights against a bird, a dragon, or a crab representing heaven, earth and sea. At the special hours of noon, 18h and 22h, the three attack him all together. There are accompanying sounds for the rumbling of earth, the blowing of the wind, and the breaking of waves. It is 4M (13 feet) high and weighs about 1 ton.
The original electronics were upgraded in 1995 but sadly there has been no funding for maintenance and the clock stopped on 1 juillet 2003. To have seen the clock striking an hour was fascinating, but the special hours were beyond description! Quel dommage.
Here is a video of 14h as he fights the bird.