This is a landscape of follies, statues and temples to the virtues. My guidebook says:
A path through the trees leads to the Surprise View - a treat for any unsuspecting visitor. William [Aislabie] created this path around 1755, but it was his daughter, Mrs Allanson, who, in the 1790s, put up a shelter with a sliding door, perhaps replacing an earlier covered seat. As visitors arrived, a servant would throw the door back, dramatically revealing the view.More on the National Trust website.