"We are waiting for the bride in the city hall."
Translation:Nous attendons la mariée dans la mairie.
In theory there is not difference between "mairie" and "hôtel de ville". However usually a mairie is in small towns and a hôtel de ville in bigger ones. For example, in my city of 25,000 inhabitants it's a mairie. For Paris, hôtel de ville refers to the huge building where the administration of Paris stands.
I do no help you a lot, i think both should be accepted.
Yes, you are absolutely right. Same where I have a house in SW france. Villages and small towns have a mairie and the big cities like Toulouse have a hotel de ville. I think this is really another example of Duo using American standard where even small towns are referred to as a 'city' and have a city hall. In England, where I live most if the time it seems only the larger towns have a functioning Town Hall these days with administration done more on a district basis at centralised District Council offices.