The walls of walled gardens can sometimes hide a secret. Sometimes, as well as providing shelter, they can also give out heat. The clue is in the chimneys that line the top of the wall. The hollow walls act as flues, drawing heat up the wall and venting at the top. This would speed up flowering and fruiting, and protect against frosts.
The photo below shows several bricks pulled out; soot can still be found in the flues, from the fires that were lit at the back of the wall.
This photo (below) shows a rather utilitarian chimney on top of the heated wall in the walled vegetable garden.
Of course, the gentry couldn't possibly put up with the sight of such workaday stacks - it might offend their delicate sensibilities. So those walls that were heated in areas where the lords and ladies might stroll had to be disguised. As rather ornate-looking urns. Those were the days.
For detailed diagrams on how some of the heating systems used in glasshouses, here's a link to a e-book on The Early Heating of Glasshouses.