In 1618, the famous (second) defenestration of Prague is generally regarded as the main event that kicked off the cluster-you-know-what that was the Thirty Years War: an incredibly bloody and destructive series of conflicts. It was the earliest set of wars I learned about where they described the death toll in the millions. Actually, it's one of the most interesting events in European history and I've spent a lot of time over the years reading about it.
Anyhow, back to throwing people out of windows. This event kicked off the Thirty Years War - and more locally, the Bohemian Rebellion of the Estates against Emperor Ferdinand II of Austria (not Australia, auto-correct!) In the two years that followed the defenestration, there were a series of conflicts between the Bohemians and Austrians that the Bohemians eventually lost decisively at the Battle of White Mountain (Bila Hora is on my list of things to see).
After this battle, Bohemia was greatly reduced politically from a kingdom to a client state of Austria. In 1621, the Bohemian lords responsible for the political, financial, and military backing of the Rebellion of the Estates were rounded up and beheaded in Prague at the Old Town Center. It was a long and bloody affair that lasted from five in the morning until one in the afternoon on June 21st, 1621.
These Bohemian lords are regarded as heroes and martyrs, having fought for freedom of religion, the rights of Bohemians, and maintaining the rule of the kingdom within Bohemia.
The plaza commemorates them with 27 crosses on the ground and a message.
According to the legend, the ghosts of the twenty-seven lords return to this place each year on June 21st and gather at the Astronomical clock. If the clock is working, it signifies the land is prospering. If the clock is not working, they return to their eternal resting places sad and dejected.