There's a crowd every hour. Everybody wants to see how Prague Orloj, or Prague Astronomical Clock, at Old Town square works. It is quite spectacular if you keep in mind that all that show is here since the 15th century.
Well, this video on YouTube would show you more.
There's one more video where you can see the in front of Prague Orloj. It's mine :) and I took it during a friends wedding. You can see it on YouTube, too.