Looking on the internet deeply has found these results:

at the city hall is the most popular phrase on the web.

ar city hall 122,100 results on the web	at the city hall 116,400,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
And Watzek had powerful friends at City Hall. He's doing well at city hall. Suspect's heading towards city hall. We just got married today at City Hall. There's a trial at City Hall. I had an apartment downtown, but home was City Hall. We borrowed aging simulators from City Hall. This guy works at City Hall.	We had a concert at the City Hall. Well, if even at the City Hall... Well, if even at the City Hall... We need an ambulance at the City Hall Station. (German National Anthem) Unity and Justice and Liberty... (German National Anthem) Unity and Justice and Liberty... Mother slept through a classical concert at the City Hall of Schafeberg. And the start of a gigantic and unique waste collection. Of course, that's not a problem for you there, at the City Hall. This is the security camera at the City Hall car park. I'll have to be at the City Hall.

ar city hall

122,100 results on the web

at the city hall

116,400,000 results on the web

More popular!

We had a concert at the City Hall.
Well, if even at the City Hall...
Well, if even at the City Hall...
We need an ambulance at the City Hall Station.
(German National Anthem) Unity and Justice and Liberty... (German National Anthem) Unity and Justice and Liberty... Mother slept through a classical concert at the City Hall of Schafeberg. And the start of a gigantic and unique waste collection.
Of course, that's not a problem for you there, at the City Hall.
This is the security camera at the City Hall car park.
I'll have to be at the City Hall.