Well, I am sure you have already walked over the Charles bridge (Karluv most). But maybe you passed very quickly through the gate on the Malá strana side. You should stop there for a minute and try to figure out what these slots on the wall are. Why are they here? You know I am a technical guy so these things intrigue me. So if you want to solve this for yourself, do not read further and try to guess what it is…
In the medieval times, there was a guard standing right here with his back to the wall. And he was equipped with a halberd (just in case you never heard about it), a sword and a knife. And he was bored most of the time. And the tower is made of sandstone. And to kill his time, here he was sharpening his tools by grinding them on the stone, not thinking about the poor tourists a couple centuries later trying to find out what was happening here…