“I don’t think I’ve ever seen it so busy!” declares my dad as we pull into the car park of Housesteads, which sits along Hadrian’s Wall, one of the best-preserved Roman forts in Britain. He’s right: the car park is rammed and the entrance to the site is unusually crowded. Tourists with backpacks sit on picnic benches eating cake from the cafe, while a dad reads aloud the noticeboards to his small children. Hadrians map This buzz is, in part, probably due to it being an exceptionally sunny day during the school holidays. But there’s another factor: Housesteads has a new, temporary art installation, part of a year-long series of…