The Rostov Kremlin is considered by many to be the best in Russia, both in terms of it’s architecture and the extent of its renovation.
Here is a page about it in TripAdvisor.
The Rostov Cathedral is one of the oldest, grandest, and most important cathedrals in Russia. Although it is open to the public, it is currently under restoration. (Trip Advisor)
This slideshow gives an idea of how magical it is.