Worth more than a quick photo. The Guildhall itself has exhibitions on Derry's history inside, free, and better than most. The building dates from 1890 and was rebuilt after bomb damage in the 1970s with its stained glass largely preserved. The square in front was the site of significant civil rights events and carries that history quietly in its proportions.
The square hosts markets and events through summer, and the Guildhall's Great Hall is occasionally open for concerts and functions. It's also the perfect place to start a walk along the city walls, the steps up to the walls begin at the north end of the square. Come on a weekday morning when it's quiet and the Gothic detail of the building is easier to read without coaches in the foreground.