I think this classical folly, The Ruin (below), was probably my favourite though. It was a clever recycling project, built around 1750 from stone from sections of the Hall pulled down during alterations. Originally it was more extensive and included a pigeon house and classical colonnade, though they were destroyed by floods in the late 18th century. I love the fact that there's a Druid sitting on the top!
Next: Tower of the Winds.