The Cathedral of Chalesm was a building used by Slartibartfast and the Campaign for Real Time as an example of the hazards of temporal manipulation. The cathedral was scheduled for demolition in order to build a new ion factory. However, due to delays in construction and a strict deadline for the start of ion production, the beginning of the project was extended so far back in time that the cathedral ended up never having been built in the first place. As a result, picture postcards of the cathedral suddenly became immensely valuable.
And thanks to time travel, just as everywhere in the world is very similar, now everywhere in time is very similar.