"The eyes are windows to the soul."
Translation:Die Augen sind Fenster zur Seele.
Yes, it is. It would make the same difference as in english "the eyes are (the) windows to the soul" where you had a more specified and restrictive meaning (if the eyes are THE windows to the soul there are no other windows, if the eyes are just windows there could be more/other windows).
There is no problem with the sentence and its translation. At first I was a little irritated, too. But the two sentences are formulated in a different manner. When the English sentence is "to the soul", then is the right translation "zur Seele". For the sentence "of the soul", the translation "der Seele" is correct and both translations make sense in German. (: