A guildhall is a large building where, in the past, a guild met. A guild was a group of people who did the same skilled work — bakers, weavers, goldsmiths — and looked after their trade together. They needed a hall: a big room, or a whole building, for meetings, feasts, and rules. Though actually, in Britain the word did not stay only in history books. Some cities still have a Guildhall, often with a capital G, and it may now work as a town hall or a place for big civic events. London's Guildhall is a famous one. So the old 'guild meeting house' and the later 'city building' sit close together, and writers sometimes stretch the word to either. It is not an everyday American word; there people more often say city hall. You will meet it in history, in travel writing about Britain, and on maps of old towns. It feels formal and a little old.