The Ha-Ha was an important part of the park, as it kept the local farmer’s livestock – ie cattle – out of the gardens so they didn’t eat all the ornamental plants! If correctly built, at over 8 foot high (including a ditch in front of it) the Ha-Ha should also keep out deer.
The recently revealed Ha-Ha, is a fine brick example, added by James Pulham when his company built the Fernery, Cascade and Folly for the (new) Lord of The Manor of Bromley, Coles Child.
It’s thought they are called ‘Ha-Ha’s because the ‘infinity’ view would lead to a surprise when the drop and ditch was revealed, here’s the explanation from the National Trust:
Originally a feature of formal French gardens of the early 18th century, the ha-ha was first described in print in 1709 by the gardening enthusiast Dezallier d’Argenville in his La Theorie et la Practique du Jardinage (The Theory and Practice of Gardening). According to d’Argenville – and his first English translator, John James – the ha-ha derived its name from the success of the optical illusion it created from a distance on viewers of the garden: the concealed ditch and wall would ‘surprise the eye coming near it, and make one cry,‘Ah! Ah!’’