Rats can enjoy just about anything. Cardboard boxes with doors cut in them and empty tissue boxes make great hideouts, as do milk cartons and clementine boxes. A roll of toilet paper can give endless hours of amusement--except, of course, for whoever gets to try to clean it off the bars of the cage, lol! Empty toilet paper and paper towel rolls are also great to play with (you can also hide treats inside them and watch the ratties go crazy getting inside). Job-lot stores are a great place to find useful items. Those open-work plastic cat balls with bells inside are fun for rats, too; just don't get them anything with catnip in it (it's supposed to be bad for rats).
The castle looks like a lot of fun. But don't spend a lot of money on it, because it's going to be gnawed. That said, however, your rats should have a ball with it.
