The Tower had been for centuries part of the walls of the old city and separated the Jewish quarter of the city from the cemeteries of the Muslims and Jews. The city walls were demolished in 1866 and as soon as Thessaloniki was liberated from the Ottoman domination in 1912, the Tower was whithewashed as a symbolic gesture of cleansing.
Today the Tower is known as White Tower and houses an imortant exhibition dedicated to the history of the city.
More historical information on http://en.wikipedia.org/wiki/White_Tower_of_Thessaloniki
and more information about the museum here: http://www.lpth.gr/en/