To be honest, when I found this postcard in my mailbox, I thought it must be somewhere in Europe :) But no, it's really in the United States!
I like it when cards have some description (it doesn't mean that I don't like using Wikipedia... I like it, but I think cards should have some description for these people, who don't use other sources, but would like to have some idea about the cards they receive) :) So, this time, I'll just copy what is written on the back side of this card:
Boldt Castle, 1000 Islands
Located on Heart Island in the St. Lawrence River, this magnificent structure was built by George Boldt for his wife at cost of over 2 million dollars in the early 1900's. It was never completed due to her death.