It will cost you a little over 10 Euros to take the elevator up to the third observation deck, where you can walk around the top with your GPS in hand, to zero in on the following coordinates:
N48 51.494 E2 17.661
Here you will find a brass plaque, and behind it a thin zip-lock back containing the Geocache log book. Because of the very thin location, this cache container will not accommodate any objects other than the log book. (Unless you want to leave: cash, cheque, or money order), so please don't be too disappointed if you cannot cram anything of fun inside the zip-lock bag.
The hiding spot is in a very high traffic tourist zone, so you may want to look at the spoiler photograph for the exact location. Although there is a physical log book at the site at the moment, the tourist zone nature of this cache may mean that it will soon turn into a virtual cache.
Feel free to E-mail me at: email@example.com (remove the xyz from the name before mailing) If you make it to the site, and let me know if you found the logbook. (I recommend leaning back against the brass plate, before reaching up to retrieve the cache.)
The wind was howling, and the rain pelting me (sideways) on the night the cache was created, so you may find the coordinates off by a some distance. (please don't fall off the tower).
By all means enjoy the view while you are up there!