The city hall is the building which a city council uses as its main offices.
They massed in front of the city hall.
...at Sheffield City Hall.
city hall in British English
noun
1.
the building housing the administrative offices of a city or municipal government
2. City Hall
3. mainly US and Canadian
a.
municipal government
b.
the officials of a municipality collectively
4. US informal
bureaucracy
City Hall in British English
the building, located in Southwark and designed by Norman Foster, housing the headquarters of the Greater London Authority and London Mayor; opened in 2002
See full dictionary entry for city hall
city hall in American English
US
1.
a building which houses the offices of a municipal government