What’s this mysterious little tower?
According to some, this is an escape exit:
Stalinkas are the residential apartment buildings, that were built during the Stalin era all over the Soviet Russia. In St. Petersburg, these buildings had an underground bomb shelter in their basement. Also, there were little towers in the middle of the courtyards. They served as an escape exit from a bomb shelter. So-called, krysatniks (rat exits) still stay tall in St. Petersburg backyards, as the reminder of the Cold War.
Others think that this is the subway vent. While, many argue that this is the old and abandoned underground pharmacy vent.
Update: This is, in fact, the escape from the underground bomb shelter.