It's actually called the Marin Civic Center, but I've always thought that it more closely resembles a Martian city. George Lucas must have thought so, too, because it supposedly was his inspiration for the Planet Naboo in his Star Wars series. In the 1970s it was notorious as the site of a courtroom shootout and subsequent murder of a judge.
The Marin Civic Center is the last work of architect Frank Lloyd Wright and the only public building that he designed. It houses administrative offices, the main county library, and the Hall of Justice. I've read that Wright, who died before the Civic Center was completed, had intended for it to be painted in the colors of the surrounding landscape -- gold, brown, and green. Somehow it ended up being pink and blue, like a baby's nursery.
If you think that it looks like a Martian city from the outside, you should see it from the inside. It's designed in six concentric oblongs: L, C, and floors 1-4. Think of a racetrack that shrinks a little as you ascend to the next level. But the oblongs are not the same size and are not stacked exactly on top of each other, so the 4th floor is skewed 'way over to the right. It's a challenge to find your way around.
There are lozenge-shaped cutouts on each floor, so you can look over the wall and down to the other levels, and large trees grow up between the levels. The domed roof stretches over it all, with large skylights you can't see from the street. On the 4th floor, the air is tropical, and the halls are so narrow that only two people can walk abreast.
A hidden gem: the main County library contains oral histories from old-time Marin County residents, and it's amazing to read them.
There is a Frank Lloyd Wright gift shop in Room 233, and a cafe that is open to the public. There are tours of the Civic Center every Wednesday at 10:30 am. Cost is $5 per person, and tours take 1 to 1.5 hours. No reservations are necessary -- meet at the gift shop. Groups of 10 people or more should phone ahead for an appointment: 415-499-7009.