Where was this filmed?

This was the same studio where they made King Kong. Remember the Skull Island village, the big wall (Which was burned down for Gone With The Wind), and the jungle with the stop motion dinosaurs and King Kong.

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The interiors were shot at RKO studios but the entire recreation of medieval Paris including a 190 ft replica of Notre Dame Cathedral was built at the RKO ranch in the San Fernando Valley. The cathedral's upper towers were actually not built but achieved by the addition of a "hanging miniature" combined with the full scale lower portion. Actual footage of the real Notre Dame was also incorporated in some scenes. The Cathedral set was re-used in many later films.

The interiors were shot at RKO studios but the entire recreation of medieval Paris including a 190 ft replica of Notre Dame Cathedral was built at the RKO ranch in the San Fernando Valley. The cathedral's upper towers were actually not built but achieved by the addition of a "hanging miniature" combined with the full scale lower portion. Actual footage of the real Notre Dame was also incorporated in some scenes. The Cathedral set was re-used in many later films.

It's now referred to as the RKO Encino Ranch.

According to the documentary on the DVD, the location of the back lot was the northwest corner of Balboa and Burbank Blvds in Encino in The San Fernando Valley. That's not quite true, though. The actual location was just to the west of there.

The location of the old lot is bordered by Oxnard Street on the north, Burbank Blvd. on the south, Louise Ave. to the west, and the western edge of the Balboa Sports Center Complex (a park) on east. It's now a collection residential streets.

From old aerial photos, it looks like the center of the the cathedral steps (which faced south) would have been on what is now Ostrom Ave, a few houses to the south of its intersection with Hatteras St.

Here's a website devoted to the RKO Encino Ranch.

http://www.retroweb.com/rko_encino_hunchback.html