The Meridian. When the shadow of the Astronomical tower crosses this line in the Big square that would signify "High Noon" A flagger in the Tower would then wave his flags and watchers in many other church towers and steeples across Prague would adjust there clocks accordingly. The flagger system was used until 1928. It was about 2 minutes and 45 seconds different then Central European time and was know as Czechoslovakian Local Time.
This is the Astronomical tower.
This is the view from the Astronomical Tower to the Meridian bottom right corner. (last two photos are from unknown photographer)