Here is the wikipedia definition of a Greek water clock:
In Greece, a water clock was known as a clepsydra (water thief). A commonly used water clock was the simple outflow clepsydra. This small earthenware vessel had a hole in its side near the base.....
Between 270 BC and 500 AD, Hellenistic (Ctesibius, Hero of Alexandria, Archimedes) and Roman horologists and astronomers were developing more elaborate mechanized water clocks. The added complexity was aimed at making the flow more constant by regulating the pressure, and at providing fancier displays of the passage of time. For example, some water clocks rang bells and gongs, while others opened doors and windows to show figurines of people, or moved pointers, and dials. Some even displayed astrological models of the universe.
The biggest achievement of the invention of clepsydrae during this time, however, was by Ctesibius with his incorporation of gears and a dial indicator to automatically show the time as the lengths of the days changed throughout the year, because of the temporal timekeeping used during his day.
In the lesser of the two references Ibn Butlan (as related in Lestrange), a Christian Arab and a physician visited the city in 1051. He says:
"In the centre of the city is the church of Al Kusiyan. It was originally the palace of Kusiyan, the king...At one of the gates of this church is a Clepsydra (Finjan) showing the hours. It works day and night continuously, twelve hours at a round, and it is one of the wonders of the world."
This is an interesting reference to the church that is sometimes called the Cassianus. We shall dwell more on this in our commentary on the Regia.
Then we have the Chinese report (as related by Hirth) written around 945 AD which considerably fleshes out the detail.
"Coming outside to the royal residence there are three large gates beset with all kinds of rare and precious stones (K18; cf. L17). On the upper floor of the second gate they have suspended a large golden scale; twelve golden balls are suspended from the scale-stick (a steelyard) by which the twelve hours of the day are shown. A human figure has been made all of gold of the size of a man standing upright, on whose side, whenever and hour has come, one of the balls will drop, the dingling sound of which makes known the divisions of the day without the slightest mistake (K19; cf. L18)."
This sounds like a fairly spectacular device. Clepsydrae were common in the ancient world, the most famous detailed for us being the still extant Tower of the Winds in Athens. However the Antioch clepsydrae would seem to leave any of these other documented water-clocks looking mediocre.