In fact I was referring to these fruit walls:
"In 1561, Swiss botanist Conrad Gessner described the effect of sun-heated walls on the ripening of figs and currants, which mature faster than when they are planted further from the wall. Gessner's observation led to the emergence of the "fruit wall" in Northwestern Europe. By planting fruit trees close to a specially built wall with high thermal mass and southern exposure, a microclimate is created that allows the cultivation of Mediterranean fruits in temperate climates, such as those of Northern France, England, Belgium and the Netherlands."
Very popular during the last grand solar minimum.
Aha! No wonder these fruit walls looked quite old and strong!