Bart, the troll guarding Dijkstra's treasure, could have been the poster boy for the latest craze among Novigrad's wealthy strata, a trend that had swept over everyone from bankers and merchants to underworld crime bosses. The fashion in question prized guards marked not just by fierce loyalty, but also by massive size, endless stamina and crushing physical strength. A well-developed intellect, on the other hand, was considered unnecessary and quite possibly detrimental to the guard's performance. To be blunt, a good watchman should be brainless enough to render any thought of betraying his master impossible.
That is why trolls, ogres and, as a last result, magic crossbreeds were highly prized for such roles.