The oldest city clock built by Hans Luter in 1535 is on the Zyt tower.  This clock is privileged to chime every hour one minute before all the other city clocks.  Unique collection of old tower clocks (1526 t0 1820) is in Lucerne’s clock tower.  Experience the huge clock face and rhythmic movements of the 9 metre pendulum.*

The website Turmuhren Luzern provides detailed information about the tower clock, the tower and the clock exhibition:

The clock-tower was the only tower whose purpose was not for defence. Thanks to its big clock-face and bell the people in the city and the boat crews out on the lake could read the accurate time. Still today the stroke of the clock-tower clock is one minute before the chime of the church bells (privilege of first stroke).
In 1579 the clock-tower was hit by lightning and on 16th August 1583 the bell and the clockwork as well as the top platform were hit again. Smoke billowed but the tower as well as the clockwork remained undamaged.

Painting on the façade
In 1511 the painting on the façade was copied for the first time by the town clerk Diebold Schilling in his chronicle. It shows two red men holding bell clappers standing on both sides of the opening in the wall and the clock-face with two giants (: wild men). The giants were the symbol of strength and power of the soldiers and mercenaries of Lucerne. In 1547 the tower was redecorated in fresco technique. In 1596 the fresco was renewed by Joseph Moser and in 1939 Karl F. Schobinger (1879-1951) designed the actual painting on the façade where the two giants still hold the coat of arms of Lucerne.
Schobinger was a pupil of Ferdinand Hodler in Geneva. From 1911 to 1914 he taught the art of painting at the academy of arts in Breslau (today: Wroclaw / PL).

More interesting information about the Zytturm clock can be found here.

*Lucerne city guide 2013