It was a little bit steep but not really very far to climb from the town up to the fortress. I think it took us 30 minutes or so from our hotel.
But we did take a sort of short cut because our hotel was at the far south side of the fortress and the fortress entrance is up the hill on the north side.
We walked along the Quai du Rempart from the bridge Pont de France and turned left in a little street called Rue de L'imprimerie (meaning the Printers street). There is a pathway up the hill starting just about from there (it is signed) and climbed that.
The tourists train rides every weekend from Easter until 11/11 (weather permitting).
And also every day in July and August.
The schedules are on signs on the bridges calles Point de Liège and Pont de France and also on the Esplanade Godefroy (which is the entrance to the fortress).
What you get is a 40 minute ride with 3 lookouts along 7 kilometres, spanning 12 centuries of history. They say it's the most beautiful ride in all of Belgium!
The more practical side to this transportation tip is that it means that you can take the little train up to the fortress instead of walking all the way up!
The contact address is:
12 Rue de Laitte, Bouillon.