The makeshift training grounds can be found in the small courtyard that is near the abandoned animal cages in Haunting Ground.
After Hewie rescues Fiona Belli from Debilitas in her guest room, they explore the courtyard and find a blocked gate that only Hewie can open by crawling through a small hole and moving the wooden crate aside.
Here, Fiona can become more familiar with Hewie and train him to follow her commands-such as attacking scarecrows, walking along a ramp, jumping over a fence and fetching a Ball.
It is vital to the player to use this area as a means of gaining Hewie's loyalty and helping him become more obedient. This is also good practice before exploring the castle more thoroughly and encountering a Stalker.
The origins of the enclosed area are unknown; but it is fairly possible that it was once used for training canine companions at some point in Belli history.