Our architectural walking tour started at a roof-top popos (Privately Owned Public Open Spaces) located next to (and accessible through) the Crocker Galeria. Unbeknownst to me, there are 68 of these semi-secret privately owned public spaces in San Francisco, most of them located downtown, as illustrated here. Most of them are not well marked, even hidden, likely intentionally, so that not too many people can find them.
As we have learned during our tour, each new development in the crowded city space has to have a popos as a way of giving back "green" park space to the city and it's inhabitants. Unfortunately, the developers obey the rule, but make it very difficult for anybody to find a popos and an access to it. Moreover, they usually have an extensive list of rules of the things that are and aren't allowed in popos, and allow access to them only during limited number of hours per week (typically Monday through Friday, 9 to 5). Regardless of all these obstacles, I am planning to skip work one day and go to visit some of them. It will be a true treasure hunt :)
One of the popos:
The entrance to the Crocker Galeria from Post St:
Inside the Crocker Galeria: