Tower Lotrscak was built in 13th century as a part of the southern fortified wall. Fourth floor was added in 19th century. Its name has roots in latin "campana latrunculorum" - thief's bell which would ring every night before the fortified town's doors would close. For more than a century (it started in 1877), there has been a cannon on the 3rd floor, which fires at noon. There was a period of 2 years starting 1991 where the cannon didn't fire because of the war in Croatia. I'm not sure if it was operational during WWI and WWII.
Other than signifying noon, it's also a reminder of a Zagreb's legend. It is said that a cannon from Lotrscak tower fired a shot exactly at noon hitting a Turkish settlement across the river Sava during their invasion of this region (16-17th ct). The cannon ball supposedly hit a platter with a turkey which was meant for pasha (a hot shot in Turkish army). Because of this "aimed" shot, it is said that is why the Turks never conquered Zagreb. :)