the "buda" part of budapest is hilly and mainly limestone. since limestone is so porous and they have so many natural springs they have a lot of caves, tunnels, etc.
underneath the castle district they have a labyrinth of sorts which you can enter from 3 different entrances that offer different experiences. our tour guide recommended one that had a wine fountain where you could bring any empty bottle and literally fill it up as many times as you wanted.
wife- "so, is it like a chocolate fountain, but with wine??"
me- "uh, i guess i'd say it was more like a water fountain, but with wine. i love you so much."