I've no idea about russian architecture - my father in law would probably know which books to read, but he's not on hand at the moment. Try googling and going to the library if you want something really real... however - personally - I'd just throw in a normal floorplan of a castle/palace and just tell the players that it looks russian - I bet the wont' think twice about the floorplans and if they fit - they'll have onion domes on their mind and make the floorplan russian as well. Especially if you furnish their fantasy with some russian dressings - you can always throw a picture or drawing of some russian style furnishings and a palace in there for extra inspiration.