The thicket is protecting the rose much as a dragon in the stories stood guard over treasure. The origin of 'treasure' is unknown but has a direct connection with Troy. The town of Troyes in France is where precious gems and metals were measured (there may be a reason why English aristos brought up to speak French had a tendency to pronounce 'r' as 'w'!).
The name Troy is connected to tree, three, true etc. and of course contains 'roi' or king. The root seems to be truare which means “circular movement around a stable centre”. Troy was originally the descriptive name for circular hillforts which were usually surrounded by concentric rings of dykes. These hillforts date back in many cases to the Neolithic period. There are ancient rock carvings which show this circular form of labyrinth leading by devious route to the centre.
In Northern Europe the treading of a maze is still called the “Game of Troy” which suggests a common characteristic between circular (or unicursal) labyrinths and circular hillforts of the ancient city of Troy; both protect something valuable at the centre.