Guanajuato
This charming old city is distinguished, among other things, by the elaborate underground thoroughfare road system. The surface streets are narrow and some have stairs and inclines, and around every bend is another glimpse of the colors of Mexico gleaming on the houses that rise up the surrounding hills (click here for more information and history).
It is worth getting a guide to help you find the nooks a crannies of this most interesting city. And, by all means, follow one of the troubadours in medieval garb one evening.