The doors to the cathedral were opened in 1879. The architect was James Renwick, an American. The Lady Chapel was designed by another American, Charles Mathews.
I think that the most unique thing about the cathedral is that it is located on 5th Avenue among the swanky shops, posh hotels, and tall modern buildings. It looks totally out of place, though it compliments its area. It creates a beautiful contrast of new and old New York.
Entrance is free and photographs are allowed as long as mass is not in progress. There is a small gift store on site.