"The bride and groom arrived at the city hall on a bike!"
Translation:Les mariés sont arrivés à la mairie à vélo !
Duo is being a bit strict here because the English has 'bike', ie one of the colloquial shortened forms of 'bicycle' (the other is 'cycle'). Vélo is correctly used for 'bike' because it is the shortened form of the 19th century word for a bicycle: 'le vélocipède', and 'bicyclette' would be wrong.
There are just under 35,000 communes in France, with a median size of 4 sq miles, and mostly rural.
As a result, most mairies are not large enough to cope with more than one wedding at a time, which is why it's a bit daft calling them "City Hall".
I do not know what the median size of the full time staff of a mairie is, but I would not be surprised if it can be counted with your fingers.