Krakow is a very old city with many buildings whose construction goes back centuries. Many old utilities can still be found in and around such buildings, some of which have nearly been forgotten. A prime example of this is the so-called torch sockets.
These round holes that can be found near the entrance of several buildings in the old town confuse many people. Some think that they are some kind of umbrella stand, or maybe a place for cannon balls. However, their use is much more utilitarian. They were there to help put out torches before one would enter an enclosed and flammable building.
The easiest to find an example of these sockets is the market hall entrance on the old square, where they line the main gate entrance. But many more examples can be found.
Know Before You Go
The sockets are freely accessible.