The "Millennium" in Silver Millennium would more likely mean "a period of universal happiness, peace or prosperity" or "a utopia" (look on Wiktionary). I believe that the 30th century is also called Silver Millennium, but don't quote me on that. I could refer to the entire kingdom of the past, but I don't really know.
Apparently, the past life mothers of the Guardian Soldiers (that is Mars, Mercury, Jupiter, and Venus) are seen in one panel in the Dream Arc of the BSSM manga - don't quote me, I do want to see a scan of that if it is true. In the anime, there would be civilians on the moon (personally, I imagine Naru, Umino, Mokoti, Ikuko, Kenji, Minako-papa, Minako-mama, and some others [not Ryou or Yuuichirou, not even Rei's grandpa or the birthparents of any of the Soldiers {including Mamoru}]), if there are civilians on the planet Venus then one could infer that there are civilians on Mars, Mercury, Jupiter, and the Moon - I'm not sure about Uranus, Neptune, Pluto, or Saturn. I would like to see the Venusian civilians if anyone has scans of them.
The castles are all named, even Serenity's castle (known as Moon Castle in Japanese), but only four are shown in total - Moon Castle and three others.